`ice-gathering-state` logic is broken in `webrtcbin`

As mentioned in #676 (comment 318038) it just skips GATHERING and became COMPLETE even before first ice candidate come to on-ice-candidate event handler. Checked on Ubuntu 19.04 + GStreamer 1.15.90 and Ubuntu 19.10 + GStreamer 1.16.1. If it required, I can prepare demo for this issue, but you can easily see it in logs with webrtcbidirectional.c if just set GST_DEBUG=webrtcbin:5