Let gnorb-gnus-check-outgoing-headers handle news

lisp/gnorb-gnus.el: gnorb-gnus-check-outgoing-headers needs not to choke
		    on news: use either To or Newsgroups, appropriately.

		    Probably this whole function should be wrapped in
		    some kind of condition-case -- it should never
		    prevent messages from being sent.