public void TestMessageMissingExtensions()
        {
            TestXmlMessage original = TestXmlMessage.CreateBuilder()
                                      .SetValid(true)
                                      .SetText("text")
                                      .SetExtension(UnittestExtrasXmltest.ExtensionText, " extension text value ! ")
                                      .SetExtension(UnittestExtrasXmltest.ExtensionMessage, new TestXmlExtension.Builder().SetNumber(42).Build())
                                      .AddExtension(UnittestExtrasXmltest.ExtensionNumber, 100)
                                      .AddExtension(UnittestExtrasXmltest.ExtensionNumber, 101)
                                      .AddExtension(UnittestExtrasXmltest.ExtensionNumber, 102)
                                      .SetExtension(UnittestExtrasXmltest.ExtensionEnum, EnumOptions.ONE)
                                      .Build();

            TestXmlMessage message = original.ToBuilder()
                                     .ClearExtension(UnittestExtrasXmltest.ExtensionText)
                                     .ClearExtension(UnittestExtrasXmltest.ExtensionMessage)
                                     .ClearExtension(UnittestExtrasXmltest.ExtensionNumber)
                                     .ClearExtension(UnittestExtrasXmltest.ExtensionEnum)
                                     .Build();

            JsonFormatWriter writer = JsonFormatWriter.CreateInstance();

            writer.WriteMessage(original);
            Content = writer.ToString();

            IMessageLite copy = JsonFormatReader.CreateInstance(Content)
                                .Merge(message.CreateBuilderForType()).Build();

            Assert.AreNotEqual(original, message);
            Assert.AreNotEqual(original, copy);
            Assert.AreEqual(message, copy);
        }