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.

Yet, if this is a good test for consistency we should have it as a separate test and test all the registered actions, not only a few of them.