mod_websocket: Clone stanza before mutating (fixes #1398) Checking for `stanza.attr.xmlns == nil` to determine if the stanza object is an actual stanza (`<message>`, `<presence>` or `<iq>` in the `jabber:client` or `jabbber:server` namespace) or some other stream element. Since this mutation is not reverted, it may leak to other places and cause them to mistreat stanzas as stream elements. Especially in cases like MUC where a single stanza is broadcast to many recipients.