Unpickle an element even if it contains ContextEntity in its attributes

Hi. I'm a user of this product and appreciate it.

I found that the connection is disconnected when I receive xml element which contains entity context in its attributes.

for example)
```
<presence to='xxxx@chat.hipchat.com/none' from='yyyy@conf.hipchat.com/&quot;Hiratara&quot;'><x xmlns='http://jabber.org/protocol/muc#user'><item affiliation='member' jid='zzz@chat.hipchat.com' role='participant'/></x></presence>
```

This patch fix the problem.

Thanks.