ejabberd -- mod_muc_occupantid: Fix handling multiple occupant-id

ejabberd team reports:

Fixed issue with handling of user provided occupant-id in messages and presences sent to muc room. Server was replacing just first instance of occupant-id with its own version, leaving other ones untouched. That would mean that depending on order in which clients send occupant-id, they could see value provided by sender, and that could be used to spoof as different sender.

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