core: Send User-Agent header in requests

This is required for the server to automatically mark
sent messages as read. See `sent_by_human` in
`zulip/zerver/models.py`.

Fixes: zulip#440

Author: sirpengi

lib/api/core.dart

@@ -1,7 +1,9 @@
 import 'dart:convert';
 import 'dart:io';
 
+import 'package:device_info_plus/device_info_plus.dart';
 import 'package:http/http.dart' as http;
+import 'package:package_info_plus/package_info_plus.dart';
 
 import '../log.dart';
 import '../model/localizations.dart';
@@ -69,6 +71,28 @@ class ApiConnection {
     }
   }
 
+  Future<void> addUserAgent(http.BaseRequest request) async {
+    final packageInfo = await PackageInfo.fromPlatform();
+    final deviceInfoPlugin = DeviceInfoPlugin();
+
+    final String applicationVersion = packageInfo.version;
+    final String systemName;
+    final String systemVersion;
+    if (Platform.isIOS) {
+      final deviceInfo = await deviceInfoPlugin.iosInfo;
+      systemName = deviceInfo.systemName;
+      systemVersion = deviceInfo.systemVersion;
+    } else if (Platform.isAndroid) {
+      final deviceInfo = await deviceInfoPlugin.androidInfo;
+      systemName = "Android";
+      systemVersion = deviceInfo.version.release;
+    } else {
+      systemName = "Desktop";
+      systemVersion = "?";
+    }
+    request.headers['User-Agent'] = 'ZulipMobile/$applicationVersion ($systemName $systemVersion)';
+  }
+
   final http.Client _client;
 
   bool _isOpen = true;
@@ -80,6 +104,7 @@ class ApiConnection {
     assert(debugLog("${request.method} ${request.url}"));
 
     addAuth(request);
+    await addUserAgent(request);
 
     final http.StreamedResponse response;
     try {