api test: Include read_by_sender in send-message tests

The one caller passes read_by_sender, so these tests are now more
representative of how the binding actually gets exercised.

Next, we'll have the binding condition something on whether or not
read_by_sender is passed, and that'll call for tests that omit it,
which we'll add.

Author: chrisbobbe

test/api/route/messages_test.dart

@@ -339,11 +339,13 @@ void main() {
       return FakeApiConnection.with_((connection) async {
         await checkSendMessage(connection,
           destination: StreamDestination(streamId, topic), content: content,
+          readBySender: true,
           expectedBodyFields: {
             'type': 'stream',
             'to': streamId.toString(),
             'topic': topic,
             'content': content,
+            'read_by_sender': 'true',
           });
       });
     });
@@ -352,10 +354,12 @@ void main() {
       return FakeApiConnection.with_((connection) async {
         await checkSendMessage(connection,
           destination: DmDestination(userIds: userIds), content: content,
+          readBySender: true,
           expectedBodyFields: {
             'type': 'direct',
             'to': jsonEncode(userIds),
             'content': content,
+            'read_by_sender': 'true',
           });
       });
     });
@@ -364,10 +368,12 @@ void main() {
       return FakeApiConnection.with_(zulipFeatureLevel: 173, (connection) async {
         await checkSendMessage(connection,
           destination: DmDestination(userIds: userIds), content: content,
+          readBySender: true,
           expectedBodyFields: {
             'type': 'private',
             'to': jsonEncode(userIds),
             'content': content,
+            'read_by_sender': 'true',
           });
       });
     });