Fix ticket #2208: regression in XMPP sending when server != host

Fix ticket #2208: regression in XMPP sending when server != host

The upstream class sets $this->basejid with host unconditionally, which wasn't previously an issue as the fulljid would always be filled in by the server at connect time before sending messages. With the new queued messaging, we need to make sure we've filled out $this->fulljid correctly without making a connection.
Now using $server if provided to build $this->basejid and $this->fulljid in the queued XMPP proxy class, so queued messages are sent correctly.