Add jabber fix from @natronkeltner

This pull request is added because of #3218. I'm going to ask @natronkeltner to, please, check and share a pcap in order to verify. We haven't a jabber server atm to test :-(

@natronkeltner: Could you share please pcat with msfdev[at]metasploit.com so we can verify the fix.

About the sleep thing commented in #3218, honestly, sounds like a weird behaviour probably targeting a slow server/network :? I couldn't say, get_once by default has a 10 seconds timeout to wait for the answer, so I don't see the real benefit from the sleep before the get_once :? Maybe we could call get_once with a bigger timeout, but honestly 10 seconds looks like good enough. So at the moment I'm avoiding the sleep fix, because I don't use to like the sleep for sync if there isn't a good reason to make it.