chrome 51 broke stuff

[fippo]

I was looking into the audiosendgoogrttmean feature and it has disappeared in Chrome >= 52. This is because https://bugs.chromium.org/p/webrtc/issues/detail?id=3377 has been fixed.

Those stats still appear but apparently not in the first report. For video this seems to be different?! This means they're undefined and bad things happen in https://github.com/opentok/rtcstats-server/blob/develop/features.js#L1583-L1584

Solution is probably to filter undefined and then calculate instead of throwing away things if the first entry is undefined