Fix calling Invitation.respond_to_reinvite() with provisional response

darcs-hash:20090330121224-58103-f19a5ca9fdb1873966aebfc722befc1fcca85e29
Fixed bug in evaluating re-INVITE SDP in session

darcs-hash:20090330121314-58103-f7974efe8acad183eed43bb967ef49edd38f8433