BUG/MINOR: mux-h1: don't forward request trailers named like server name header

BUG/MINOR: mux-h1: don't forward request trailers named like server name header

With "http-send-name-header <name>", the H1 mux skips any request header
with the same name and adds its own with the server's name. But request
trailers were not filtered, so a client sending a chunked request with a
trailer "X-Srv: evil" (with "http-send-name-header X-Srv") had it forwarded
after the "x-srv: s1" header added by haproxy. Servers merging trailers into
the headers may then see the client's value. Let's skip such trailers, like
it is done for headers.

"option http-drop-request-trailers" may be used as a workaround.

This patch could be backported to all supported versions.