Expose response class

This is a small clean-up to utilize the `ResponseClass` that is already available on the Request object.  This allows developers to define their own request factory with a new response class and have it used everywhere.

This is an attempt to help with #1256

As was suggested in the ticket, it might make more sense to just create a `set_response_factory` API and a `response_factory` kwarg of the configurator, but since the ResponseClass is already there it doesn't hurt to utilize it.  I can't think of a time you would want to create a response when you don't have a request which is why I opted to take this route instead.
`registry` undefined, breaks Travis.