Fix updating custom smileys

If you create a custom smiley and you send it to someone, then you update the
smiley picture and you send it again to the same person, it does not show the updated
smiley on their end, instead it shows the old picture.

Testing Done:
Updated custom smileys in Jabber on Windows and Linux and they updated accordingly.

Bugs closed: PIDGIN-17153

Reviewed at https://reviews.imfreedom.org/r/1477/