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 would maybe move this to be the actual test method, with `@Test` annotation, and have either an abstract setup method to be implemented by subclasses or even use junit annotations like `@Before` if possible in the subclasses.