diff --git a/lib/src/body.dart b/lib/src/body.dart
index e3589d0326..89f968579c 100644
--- a/lib/src/body.dart
+++ b/lib/src/body.dart
@@ -8,6 +8,9 @@ import 'dart:convert';
 import 'package:async/async.dart';
 import 'package:collection/collection.dart';
 
+import 'empty_body.dart';
+import 'utils.dart';
+
 /// The body of a request or response.
 ///
 /// This tracks whether the body has been read. It's separate from [Message]
@@ -27,6 +30,9 @@ class Body {
   /// determined efficiently.
   final int contentLength;
 
+  /// An empty stream for use with empty bodies.
+  static const _emptyStream = const Stream.empty();
+
   Body._(this._stream, this.encoding, this.contentLength);
 
   /// Converts [body] to a byte stream and wraps it in a [Body].
@@ -36,25 +42,23 @@ class Body {
   /// used to convert it to a [Stream<List<int>>].
   factory Body(body, [Encoding encoding]) {
     if (body is Body) return body;
+    if (body == null) {
+      return const EmptyBody();
+    }
 
     Stream<List<int>> stream;
     int contentLength;
-    if (body == null) {
-      contentLength = 0;
-      stream = new Stream.fromIterable([]);
-    } else if (body is String) {
-      if (encoding == null) {
-        var encoded = UTF8.encode(body);
-        // If the text is plain ASCII, don't modify the encoding. This means
-        // that an encoding of "text/plain" will stay put.
-        if (!_isPlainAscii(encoded, body.length)) encoding = UTF8;
-        contentLength = encoded.length;
-        stream = new Stream.fromIterable([encoded]);
-      } else {
-        var encoded = encoding.encode(body);
-        contentLength = encoded.length;
-        stream = new Stream.fromIterable([encoded]);
-      }
+
+    encoding ??= UTF8;
+
+    if (body is Map) {
+      body = mapToQuery(body, encoding: encoding);
+    }
+
+    if (body is String) {
+      var encoded = encoding.encode(body);
+      contentLength = encoded.length;
+      stream = new Stream.fromIterable([encoded]);
     } else if (body is List) {
       contentLength = body.length;
       stream = new Stream.fromIterable([DelegatingList.typed(body)]);
@@ -68,19 +72,6 @@ class Body {
     return new Body._(stream, encoding, contentLength);
   }
 
-  /// Returns whether [bytes] is plain ASCII.
-  ///
-  /// [codeUnits] is the number of code units in the original string.
-  static bool _isPlainAscii(List<int> bytes, int codeUnits) {
-    // Most non-ASCII code units will produce multiple bytes and make the text
-    // longer.
-    if (bytes.length != codeUnits) return false;
-
-    // Non-ASCII code units between U+0080 and U+009F produce 8-bit characters
-    // with the high bit set.
-    return bytes.every((byte) => byte & 0x80 == 0);
-  }
-
   /// Returns a [Stream] representing the body.
   ///
   /// Can only be called once.
@@ -94,3 +85,12 @@ class Body {
     return stream;
   }
 }
+
+class _EmptyBody implements Body {
+  const _EmptyBody();
+
+  Encoding get encoding => UTF8;
+  int get contentLength => 0;
+
+  Stream<List<int>> read() => const Stream<List<int>>.empty();
+}
diff --git a/lib/src/empty_body.dart b/lib/src/empty_body.dart
new file mode 100644
index 0000000000..d811b78276
--- /dev/null
+++ b/lib/src/empty_body.dart
@@ -0,0 +1,25 @@
+// Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'dart:async';
+import 'dart:convert';
+
+import 'body.dart';
+
+/// An empty request body.
+///
+/// Used as an optimization when a `null` body is used.
+class EmptyBody implements Body {
+  /// An empty body is not encoded.
+  Encoding get encoding => null;
+
+  /// An empty body has no length.
+  int get contentLength => 0;
+
+  /// Creates an instance of [EmptyBody].
+  const EmptyBody();
+
+  /// Returns an empty [Stream] representing the body.
+  Stream<List<int>> read() => const Stream<List<int>>.empty();
+}
diff --git a/lib/src/message.dart b/lib/src/message.dart
index 96608dda66..d78d22eeb1 100644
--- a/lib/src/message.dart
+++ b/lib/src/message.dart
@@ -5,7 +5,6 @@
 import 'dart:async';
 import 'dart:convert';
 
-import 'package:collection/collection.dart';
 import 'package:http_parser/http_parser.dart';
 
 import 'body.dart';
@@ -61,12 +60,27 @@ abstract class Message {
       {Encoding encoding,
       Map<String, String> headers,
       Map<String, Object> context})
-      : this._(new Body(body, encoding), headers, context);
+      : this._forwardToContentType(body, encoding,
+            _determineMediaType(body, encoding, headers), headers, context);
 
-  Message._(Body body, Map<String, String> headers, Map<String, Object> context)
+  Message.withContentType(Body body, MediaType contentType,
+      Map<String, String> headers, Map<String, Object> context)
+      : this._fromValues(
+            body, _adjustHeaders(headers, body, contentType), context);
+
+  Message._forwardToContentType(body, Encoding encoding, MediaType contentType,
+      Map<String, String> headers, Map<String, Object> context)
+      : this.withContentType(
+            new Body(body, encoding), contentType, headers, context);
+
+  /// Creates a new [Message].
+  ///
+  /// This constructor should be used when no computation is required for the
+  /// [body], [headers] or [context].
+  Message._fromValues(
+      Body body, Map<String, String> headers, Map<String, Object> context)
       : _body = body,
-        headers = new HttpUnmodifiableMap<String>(_adjustHeaders(headers, body),
-            ignoreKeyCase: true),
+        headers = new HttpUnmodifiableMap<String>(headers, ignoreKeyCase: true),
         context =
             new HttpUnmodifiableMap<Object>(context, ignoreKeyCase: false);
 
@@ -144,55 +158,119 @@ abstract class Message {
   /// changes.
   Message change(
       {Map<String, String> headers, Map<String, Object> context, body});
-}
 
-/// Adds information about encoding to [headers].
-///
-/// Returns a new map without modifying [headers].
-Map<String, String> _adjustHeaders(Map<String, String> headers, Body body) {
-  var sameEncoding = _sameEncoding(headers, body);
-  if (sameEncoding) {
-    if (body.contentLength == null ||
-        getHeader(headers, 'content-length') == body.contentLength.toString()) {
-      return headers ?? const HttpUnmodifiableMap.empty();
-    } else if (body.contentLength == 0 &&
-        (headers == null || headers.isEmpty)) {
-      return const HttpUnmodifiableMap.empty();
+  /// Determines the media type based on the [headers], [encoding] and [body].
+  static MediaType _determineMediaType(
+          body, Encoding encoding, Map<String, String> headers) =>
+      _headerMediaType(headers, encoding) ?? _defaultMediaType(body, encoding);
+
+  static MediaType _defaultMediaType(body, Encoding encoding) {
+    //if (body == null) return null;
+
+    var parameters = {'charset': encoding?.name ?? UTF8.name};
+
+    if (body is String) {
+      return new MediaType('text', 'plain', parameters);
+    } else if (body is Map) {
+      return new MediaType('application', 'x-www-form-urlencoded', parameters);
+    } else if (encoding != null) {
+      return new MediaType('application', 'octet-stream', parameters);
     }
+
+    return null;
   }
 
-  var newHeaders = headers == null
-      ? new CaseInsensitiveMap<String>()
-      : new CaseInsensitiveMap<String>.from(headers);
-
-  if (!sameEncoding) {
-    if (newHeaders['content-type'] == null) {
-      newHeaders['content-type'] =
-          'application/octet-stream; charset=${body.encoding.name}';
-    } else {
-      var contentType = new MediaType.parse(newHeaders['content-type'])
-          .change(parameters: {'charset': body.encoding.name});
-      newHeaders['content-type'] = contentType.toString();
-    }
+  static MediaType _headerMediaType(
+      Map<String, String> headers, Encoding encoding) {
+    var contentTypeHeader = getHeader(headers, 'content-type');
+    if (contentTypeHeader == null) return null;
+
+    var contentType = new MediaType.parse(contentTypeHeader);
+    var parameters = {
+      'charset':
+          encoding?.name ?? contentType.parameters['charset'] ?? UTF8.name
+    };
+
+    return contentType.change(parameters: parameters);
   }
 
-  if (body.contentLength != null) {
-    var coding = newHeaders['transfer-encoding'];
-    if (coding == null || equalsIgnoreAsciiCase(coding, 'identity')) {
-      newHeaders['content-length'] = body.contentLength.toString();
+  /// Adjusts the [headers] to include information from the [body].
+  ///
+  /// Returns a new map without modifying [headers].
+  ///
+  /// The following headers could be added or modified.
+  /// * content-length
+  /// * content-type
+  static Map<String, String> _adjustHeaders(
+      Map<String, String> headers, Body body, MediaType contentType) {
+    var modified = <String, String>{};
+
+    var contentLengthHeader = _adjustContentLengthHeader(headers, body);
+    if (contentLengthHeader.isNotEmpty) {
+      modified['content-length'] = contentLengthHeader;
+    }
+
+    var contentTypeHeader = _adjustContentTypeHeader(headers, contentType);
+    if (contentTypeHeader.isNotEmpty) {
+      modified['content-type'] = contentTypeHeader;
     }
+
+    if (modified.isEmpty) {
+      return headers ?? const HttpUnmodifiableMap.empty();
+    }
+
+    var newHeaders = headers == null
+        ? new CaseInsensitiveMap<String>()
+        : new CaseInsensitiveMap<String>.from(headers);
+
+    newHeaders.addAll(modified);
+
+    return newHeaders;
   }
 
-  return newHeaders;
-}
+  /// Checks the `content-length` header to see if it requires modification.
+  ///
+  /// Returns an empty string when no modification is required, otherwise it
+  /// returns the value to set.
+  ///
+  /// If there is a contentLength specified within the [body] and it does not
+  /// match what is specified in the [headers] it will be modified to the body's
+  /// value.
+  static String _adjustContentLengthHeader(
+      Map<String, String> headers, Body body) {
+    var bodyContentLength = body.contentLength ?? -1;
 
-/// Returns whether [headers] declares the same encoding as [body].
-bool _sameEncoding(Map<String, String> headers, Body body) {
-  if (body.encoding == null) return true;
+    if (bodyContentLength >= 0) {
+      var bodyContentHeader = bodyContentLength.toString();
 
-  var contentType = getHeader(headers, 'content-type');
-  if (contentType == null) return false;
+      if (getHeader(headers, 'content-length') != bodyContentHeader) {
+        return bodyContentHeader;
+      }
+    }
+
+    return '';
+  }
 
-  var charset = new MediaType.parse(contentType).parameters['charset'];
-  return Encoding.getByName(charset) == body.encoding;
+  /// Checks the `content-type` header to see if it requires modification.
+  ///
+  /// Returns an empty string when no modification is required, otherwise it
+  /// returns the value to set.
+  ///
+  /// If the contentType within [body] is different than the one specified in the
+  /// [headers] then body's value will be used. The [headers] were already used
+  /// when creating the body's contentType so this will only actually change
+  /// things when headers did not contain a `content-type`.
+  static String _adjustContentTypeHeader(
+      Map<String, String> headers, MediaType contentType) {
+    var headerContentType = getHeader(headers, 'content-type');
+    var bodyContentType = contentType?.toString();
+
+    // Neither are set so don't modify it
+    if ((headerContentType == null) && (bodyContentType == null)) {
+      return '';
+    }
+
+    // The value of bodyContentType will have the overridden values so use that
+    return headerContentType != bodyContentType ? bodyContentType : '';
+  }
 }