RequestFilter adjustments

Make the API more useful by allowing to override the response in pre-request filter so we can build rate-limiting, caching, 304 responses and so on.

Just return non-true from the pre-request handler, and the framework will try to parse it as a response value. Return SS_HTTPResponse to get that delivered to the user.

Needs some more testing - I've only checked against https://github.com/mateusz/controllerpolicy
Most important consideration when breaking API between minor releases is: will the change be immediately obvious, or will everything look like it's still working, but be introducing subtile errors into your logic.

In this case, because of the typing of the arguments, trying to use the old API will trigger an error, so it's probably OK.