Drop headline messages sent to offline resources

Don't bounce an error when a message of type "headline" is sent to an
unavailable resource.  This is consistent with how headline messages
sent to the bare JID of an offline user are dropped, and it avoids a
presence leak.

src/ejabberd_sm.erl

@@ -520,6 +520,7 @@ do_route(From, To, #xmlel{} = Packet) ->
 			<<"chat">> -> route_message(From, To, Packet, chat);
 			<<"normal">> -> route_message(From, To, Packet, normal);
 			<<"">> -> route_message(From, To, Packet, normal);
+			<<"headline">> -> ok;
 			<<"error">> -> ok;
 			_ ->
 			    ErrTxt = <<"Incorrect message type">>,