Add support for Protobuf format response and test

[salamer]

Gin now have the protobufBinding function to check the request format, but didn't have a protobuf response function like c.YAML().
In our company ByteDance, the largest internet company using golang in China, we use gin to transfer Protobuf instead of Json, we have to write some internal library to make some wrappers to achieve that, and the code is not elegant. So we really want such a feature.

The last pull request was blocked because I forgot to write the unit test. This time I have added the unit test, using the protobuf in the grpc example.

[thinkerou]

duplicated #1479
@salamer If you change the pull request, please commit not new pull request. you should use git command.