Invoke {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 are not 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 `#[]`.
Merge pull request #91 from gsamokovarov/call-responses

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