Need to enable support for xhtml under XIFF.

I can't see this patch to see if it has been fixed but our team found this issue with XIFF. Here is the scenario:

1. Revoke a user's voice in a room
2. Have the user send an XHTML message
3. XIFF blows up with an error message

We found the error and explain it in this post - http://www.igniterealtime.org/community/message/201064#201064. At the end of the post I've attached a version that has worked for a couple of users.

Patch previously applied and seems to be working.