Response regression bug fix and Request refactoring.

There's 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 and it stopped plugin responses working. This commit should be merged asap.

I also renamed `ResponseInternal` to just `Response`, cause it's not `Internal` any more.

The last commit modifies `ScriptableResponse` and makes plugin use the Scala class,  `Response`,  directly. It has quite the same purpose with d625ca0cb7637cd8bf6b0aedb2cce39724d49744.