Fix xmpp invite bug.

We use the `xmpp-ftw-muc`  met issue, I found the error. 

I think the reveived message should be like this

Details in XEP-0045
````XML
<message
    from='coven@chat.shakespeare.lit'
    id='nzd143v8'
    to='hecate@shakespeare.lit'>
  <x xmlns='http://jabber.org/protocol/muc#user'>
    <invite from='crone1@shakespeare.lit/desktop'>
      <reason>
        Hey Hecate, this is the place for all good witches!
      </reason>
    </invite>
    <password>cauldronburn</password>
  </x>
</message>
```

PLZ check , and merge.