BUG/MEDIUM: h2: properly account for DATA padding in flow control

BUG/MEDIUM: h2: properly account for DATA padding in flow control

Recent fixes made to process partial frames broke the flow control on
DATA frames, as the padding is not considered anymore, only the actual
data is. Let's simply take account of the padding once the transfer
ends. The probability to meet this bug is low because, when used, padding
is small and it can require a large number of padded transfers before the
window is completely depleted.

Thanks to user klzgrad for reporting this bug and confirming the fix.

This fix must be backported to 1.8.