BUG/MINOR: mux-h2: properly ignore R bit in WINDOW_UPDATE increments

BUG/MINOR: mux-h2: properly ignore R bit in WINDOW_UPDATE increments

The window size increments are 31 bits and the topmost bit is reserved
and should be ignored, however it was not masked, so a peer sending it
set would emit a negative value which could actually reduce the current
window instead of increasing it. Note that the window cannot reach zero
as there's already a test for this, but transfers could slow down to
the same speed as if an initial window of just a few bytes had been
advertised. Let's just mask the reserved bit before processing.

This should be backported to all stable versions.