Close the unused request body when serving up a custom Rack::Preview response

The current rack-preview gem fails to close the original response body when sending a custom response to Safari Top Sites, which can cause the server to return a "deadlock; recursive locking" error on a subsequent request.

See: https://github.com/rack/rack/issues/495, http://johnbintz.github.io/blog/2012/03/05/closing-given-body-in-rack-middleware/