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.
I find this method a bit confusing cause it sets stuff on the argument. I'd maybe add a `putHeaders` method to `ActionRequest` and make sure we can do it on a single line where we need to do it. The argument of the `putHeader` method could either be the `Headers` object itself, or we could make this `Iterable` or something and pass the key-vaue pairs to the action request.