Response tweaks.
Response tweaks.

- Add file header and copyright.
- Make the class JsonSerializable.
- Add Response::globalCookies() and Response::globalHeaders().
- Add Response::current() to get the current response.
- Add Respose->contentTypeFromAccept().
- Add Response::statusMessage().
- Rename Response->cookies() and Response->headers().
- Add Response::splitHeaders().
- Add Response::normalizeHeader().
- Add Response->flush() and Response->flushHeaders().
- Add Response::create().
vanilla/garden