Allow multiple resources for the same JID in a MUC room

Implementation of esl/MongooseIM#240
@arcusfelis @pianiel 
MongooseIM will not allow two different sessions with the same LJID, so the only case when this check can be true is when the same session requests room join twice. Is it even possible in fact? Probably this check won't even be executed when session sends another presence to the room.
Maybe we just need to check that From `luser` and `lserver` matches any of the session list members' `luser` and `lserver`?