wrong treatment of video-audio offers

[fippo]

reported by @dagingaa.
If you call addTrack(audioTrack), addTrack(videoTrack) and then set an offer containing a video mline followed by an audio mline the answer will be wrong. This happens with offers from safari, other browsers
Fiddle with repro data:
https://jsfiddle.net/mu6kcxrw/1/

There is a test for a similar issue here ('keeps the order from the remote offer') but it did not catch this behaviour, possibly due to the assertion being too weak.

[fippo]

seems fixed in master (future-but-will-never-happen-2.x) but not the 1.x version adapter uses :-|