Using the stable branch of rails 4.1
Fix build_response on rails 4.1

On the rails 4.1 build_response[1] receives one arg
on the 4.1.0.beta1[2] this arg is not present.

With this approach we set the default value to opts, to
this works correctly with 4.1.

I really dont know this is the better approach, but
fixed my broken specs =p

[1]: https://github.com/rails/rails/blob/4-1-stable/actionpack/lib/action_controller/test_case.rb#L642
[2]: https://github.com/rails/rails/blob/v4.1.0.beta1/actionpack/lib/action_controller/test_case.rb