conncheck.c: Fix negotiation problem

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)

agent/conncheck.c

@@ -2946,7 +2946,7 @@ static CandidateCheckPair *priv_add_peer_reflexive_pair (NiceAgent *agent, guint
   else
     pair->priority = nice_candidate_pair_priority (pair->remote->priority,
         pair->local->priority);
-  pair->nominated = FALSE;
+  pair->nominated = parent_pair->nominated;
   pair->prflx_priority = ensure_unique_priority (stream, component,
       peer_reflexive_candidate_priority (agent, local_cand));
   nice_debug ("Agent %p : added a new peer-discovered pair with "