BUG/MINOR: mux-h2: make it possible to set the error code on an already closed stream

BUG/MINOR: mux-h2: make it possible to set the error code on an already closed stream

When sending RST_STREAM in response to a frame delivered on an already
closed stream, we used not to be able to update the error code and
deliver an RST_STREAM with a wrong code (e.g. H2_ERR_CANCEL). Let's
always allow to update the code so that RST_STREAM is always sent
with the appropriate error code (most often H2_ERR_STREAM_CLOSED).

This should be backported to 1.9 and possibly to 1.8.