Response regression bug fix and more refactoring.

There was a regression bug in d625ca0cb7637cd8bf6b0aedb2cce39724d49744, which has been fixed with the first commit of this PR. I'm afraid I'd not modified the rest part where a response is processed, and because of it plugin responses didn't work. The first commit should be merged asap.

I also renamed `ResponseInternal` to just `Response`, cause it's not `Internal` any more.

@sgkim126 @kimchoco, I'm sorry for the regression bug and please review this asap.