Expose the xml for the last request submitted

* When an exception (e.g. timeout) is raised, a response object containing
  the request xml is not returned - this is useful when submitting
  support requests to OpenSRS

* I have added it to the Server class, as it doesn't feel like a good fit anywhere else, given that, we don't have a response if something heads south. If you would prefer, I am happy to have it cleared after the request is successful - so that it is only available on an exception being raised?