We don't currently gather the XMPP user's capabilities from their presence updates; I suspect we assume they support the same Jingle dialect we do.
At the moment we assume the XMPP client supports the current versions of Jingle, Jingle-RTP and raw-UDP. It'll do for now.
conflating this with Bug #26215 since it makes no sense to fix one and not the other
*** This bug has been marked as a duplicate of bug 26215 ***