Added an option to add arbitrary headers to the client requests

The headers are key/value pairs defined in the settings under the `request.headers` namespace.

Also changed test infrastructure's so it'd be possible to define the additional settings for transport clients.
Regarding the sanity check for actions I agree with you that it's a good test, and if we both think it's valid we should run it against all actions, not only the ones tested on this test. Also thinking if we really need reflection on this specific test, cause you could just pass in instances of the actions instead of the class name (unless I'm missing something).