MINOR: mux-h2: Set REFUSED_STREAM error to reset a stream if no data was never sent

MINOR: mux-h2: Set REFUSED_STREAM error to reset a stream if no data was never sent

According to the H2 spec (see #8.1.4), setting the REFUSED_STREAM error code
is a way to indicate that the stream is being closed prior to any processing
having occurred, such as when a server-side H1 keepalive connection is closed
without sending anything (which differs from the regular error case since
haproxy doesn't even generate an error message). Any request that was sent on
the reset stream can be safely retried. So, when a stream is closed, if no
data was ever sent back (ie. the flag H2_SF_HEADERS_SENT is not set), we can
set the REFUSED_STREAM error code on the RST_STREAM frame.

This patch may be backported to 1.9.