Invoke Rack:::{blacklisted,throttled}_response with #call

I have a response which is a class. While I can still have my class
implement `#[]`, it does look a bit off. On the other side, having
objects responding to `#call` that aren't procs is pretty common.

So I propose to invoke the responses with `#call` to let users override
it with response objects, that respond to `#call` instead of `#[]`.