MUC: Strip tags with MUC-related namespaces from private messages (fixes #1427)

Prevents duplication since it adds another <{muc#user}x> here

plugins/muc/muc.lib.lua

@@ -779,6 +779,7 @@ function room_mt:handle_message_to_occupant(origin, stanza)
 		return true;
 	end
 	log("debug", "%s sent private message stanza to %s (%s)", from, to, o_data.jid);
+	stanza = muc_util.filter_muc_x(st.clone(stanza));
 	stanza:tag("x", { xmlns = "http://jabber.org/protocol/muc#user" }):up();
 	stanza.attr.from = current_nick;
 	self:route_to_occupant(o_data, stanza)