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.
> We don't assume that all actions have the constants, only the ones that we register. This is something that makes me think using mock requests, actions and response would simplify things, as I wrote above.

The actions we register are the public actions we're testing, so it still holds (and I still think it's a good sanity check for consistency). 

I don't see how mock req, res and actions will help you to test further than this test already does. The same execution path and validation will be applied, except instead of a set of existing actions with a mock action.