BUG/MEDIUM: mux-h1: Be able to handle trailers when C-L header was specified

The commit 2eb5243e7 ("BUG/MEDIUM: mux-h1: Set outgoing message to DONE when
payload length is reached") introduced a regression. An internal error is
reported when we try to forward a message with trailers while the
content-length header was specified. Indeed, this case does not exist for H1
messages but it is possible in H2.

This patch should solve the issue #1684. It must be backported as far as
2.4.
1 file changed