Fix bad accounting of HTTP/2 windows
Fix bad accounting of HTTP/2 windows

The previous code was incorrectly substracting the maximum
frame size we could send when the data we were actually sending
was much lower.

kivra/cowboy,ninenines/cowboy,K2InformaticsGmbH/cowboy,bsmr-erlang/cowboy,hairyhum/cowboy,CrankWheel/cowboy,turtleDeng/cowboy