BUG/MINOR: mux-h1: Fix data skipping for bodyless responses

BUG/MINOR: mux-h1: Fix data skipping for bodyless responses

When payload is received for a bodyless response, for instance a response to
a HEAD request, it is silently skipped. Unfortunately, when this happens,
the end of the message is not properly handled. The response remains in the
MSG_DATA state (or MSG_TRAILERS if the message is chunked). In addition,
when a zero-copy is possible, the data are not removed from the channel
buffer and the H1 connection is killed because an error is then triggered.

To fix the bug, the zero-copy is disabled for bodyless responses. It is not
a problem because there is no copy at all. And the last block (DATA or EOT)
is now properly handled.

This bug was introduced by the commit e5596bf53 ("MEDIUM: mux-h1: Don't emit
any payload for bodyless responses").

This fix is specific for 2.4. No backport needed.