Don't return error for blocked MUC messages

If a message stanza is blocked as per XEP-0016 or XEP-0191, return an
error only if the type of the blocked message is "normal" or "chat".
This makes sure users won't be kicked from MUC rooms when blocking other
participants.

Closes #897.

src/ejabberd_c2s.erl

@@ -1636,6 +1636,8 @@ handle_info({route, From, To,
 					 deny ->
                                                case xml:get_attr_s(<<"type">>, Attrs) of
                                                    <<"error">> -> ok;
+                                                   <<"groupchat">> -> ok;
+                                                   <<"headline">> -> ok;
                                                    <<"result">> -> ok;
                                                    _ ->
                                                        Err =