Pass collection factories the request handler rather than path_kwargs.

This will allow servers to retrieve the collection owner_id from request headers (via ``handler.request.headers`` (like they should) or via ``handler.path_kwargs`` (like the tests and go-store-service do right now).
It's not vital because it's never called. I made a helper function that raises if called to make it clearer which tests are expected to call their collection factory stubs and which aren't.