Make return of response_cb explicit rather than implicit.

This bit me earlier this week when I wasn't paying enough attention and
included some logic in my call() method *after* calling response_cb().

There are other explicit returns in the code samples here.  I think it
wouldn't hurt to make it obvious that response_cb()'s return value needs
to be returned by the Middleware.
Merge pull request #479 from oalders/master

Make return of response_cb explicit rather than implicit.