BUG/MEDIUM: mux-fcgi: Be sure to never set EOM flag on an empty HTX message

BUG/MEDIUM: mux-fcgi: Be sure to never set EOM flag on an empty HTX message

This bug was already fixed at many places (stats, promex, lua) but the FCGI
multiplexer is also affected. When there is no content-length specified in
the response and when the END_REQUEST record is delayed, the response may be
truncated because an abort is erroneously detected. If the connection is not
closed because "keep-conn" option is set, the response is aborted at the end
of the server timeout.

This bug is a design issue with the HTX. It should be addressed. But it will
probably not be possible to backport them as far as 2.4. So, for now, the
only solution is to explicitly add an EOT block with the EOM flag in this
case.

This patch should fix the issue #1682. It must be backported as far as 2.4.