generation parameter is required in candidate

I noticed that strophe.jingle doesn't necessarily specify a "generation" parameter in candidateToJingle (in my case I believe the "lines" parameter sent to that function originates from the ...session.js peerconnection.onicecandidate).

Adding this will default "generation" to 0, but overwrite if it was passed along after all.

Clients like Empathy (using telepathy-gabble) failed to receive incoming call stanzas from strophe.jingle due to this, giving the error "could not parse any of the given candidates" as can be seen in its source code: http://cgit.collabora.com/git/freedesktop.org-mirror/telepathy/telepathy-gabble.git/tree/src/jingle-transport-iceudp.c?id=21a463f98c34ede8abefc02e38fbfcc2ca00c806#n327