sip_session: use lookup_routes_for_sip_uri properly

darcs-hash:20090330133304-b2873-5ba21e88cba87ca9cbf73f0ddf9f9033ee958e7a
Fix SDP generation when adding a stream in Session

darcs-hash:20090330134644-58103-cb5f3bdcd18d77cb8fe001e3926ec957edc6812c