increase test coverage

csharp/src/Google.Protobuf.Test/CodedOutputStreamTest.cs

@@ -242,6 +242,8 @@ namespace Google.Protobuf
                 output.Flush();
                 Assert.AreEqual(rawBytes, rawOutput.ToArray());
             }
+
+            // TODO: test for different chunks sizes and IBufferWriter...
         }
         
         [Test]

csharp/src/Google.Protobuf.Test/ExtensionSetTest.cs

@@ -34,6 +34,8 @@ namespace Google.Protobuf
             message.SetExtension(OptionalBoolExtension, true);
             var serialized = message.ToByteArray();
 
+            MessageParsingHelpers.AssertWritingMessage(message);
+
             MessageParsingHelpers.AssertReadingMessage(
                 TestAllExtensions.Parser.WithExtensionRegistry(new ExtensionRegistry() { OptionalBoolExtension }),
                 serialized,

csharp/src/Google.Protobuf.Test/WellKnownTypes/WrappersTest.cs

@@ -175,6 +175,8 @@ namespace Google.Protobuf.WellKnownTypes
             var message = new RepeatedWellKnownTypes { Int32Field = { 5, 0 } };
             var actualBytes = message.ToByteArray();
             Assert.AreEqual(expectedBytes, actualBytes);
+
+            MessageParsingHelpers.AssertWritingMessage(message);
         }
 
         [Test]