Added it to both 8.0 and 8.1. It's in use in my Socket.IO implementation, and so far it passes all tests.
Since testing it involves using a webbrowser as counterpart, my tests are sadly not comprehensive.
Arne, could you check the code and see if it meets your criteria on the proposed visible interfaces?
There is a little bit of trickery involved in the rsv bits. In order to know if to decompress, we need to know the RSV1 bit on the *first* frame part of the larger frameset.