BUG/MEDIUM: mux-h1: Explicitly skip request's C-L header if not set originally

BUG/MEDIUM: mux-h1: Explicitly skip request's C-L header if not set originally

Commit f89ba27caa ("BUG/MEDIUM: mux-h1; Ignore headers modifications about
payload representation") introduced a regression. The Content-Length is no
longer sent to the server for requests without payload but with a
'Content-Lnegth' header explicitly set to 0, like POST request with no
payload. It is of course unexpected. In some cases, depending on the server,
such requests are considered as invalid and a 411-Length-Required is returned.

The above commit is not directly responsible for the bug, it only reveals a too
lax condition to skip the 'Content-Length' header of bodyless requests. We must
only skip this header if none was originally found, during the parsing.

This patch should fix the issue #2386. It must be backported to 2.9.