Add test

Author: nareddyt

src/mutator_test.cc

@@ -293,6 +293,11 @@ const char kOptionalInDeepAnyFields[] = R"(
   }
 )";
 
+const char kUnknownField[] = R"(
+  optional_bool: true
+  unknown_field: "test unknown field"
+)";
+
 class TestMutator : public Mutator {
  public:
   explicit TestMutator(bool keep_initialized,
@@ -668,6 +673,12 @@ TYPED_TEST(MutatorTypedTest, Serialization) {
   }
 }
 
+TYPED_TEST(MutatorTypedTest, UnknownSerialization) {
+  typename TestFixture::Message parsed;
+  EXPECT_TRUE(ParseTextMessage(kUnknownField, &parsed));
+  EXPECT_NE(SaveMessageAsText(parsed), kUnknownField);
+}
+
 TYPED_TEST(MutatorTypedTest, DeepRecursion) {
   typename TestFixture::Message message;
   typename TestFixture::Message* last = &message;