parsers: improve xml:lang handling

This is still far from being a full fix. Still, we can avoid the lang
attribute (by allowing discarding the `xml:lang` value via the feature
introduced a couple commits ago) and we can avoid emitting `xml:lang=""`
where no language was previously set using the mechanism introduced for
<message/> in 5172fb5e.

tokio-xmpp/examples/echo_component.rs

@@ -73,7 +73,7 @@ fn make_presence(from: Jid, to: Jid) -> Presence {
     presence.show = Some(PresenceShow::Chat);
     presence
         .statuses
-        .insert(String::from("en"), String::from("Echoing messages."));
+        .insert(Lang::from("en"), String::from("Echoing messages."));
     presence
 }