conncheck.c: Fix negotiation problem

I inherited this patch to libnice from the previous team members; not sure if it still makes sense so I'm opening this to at least document it and open a discussion. The reasons given in the original commit message make sense to me but I don't really have a good grasp on all the states that libnice passes through during candidate processing.

Original commit message:

When a peer reflexive candidate is discovered, it is added to the list of connection checks, but is is not nominated by default.

This causes a problem if parent pair is already nominated and the remote peer does not send more requests because it has already selected a candidate. (i.e.: This is the case of WebRtc in Firefox)