BUG/MINOR: mux-h1/mux-fcgi: Sanitize TE header to only send "trailers"

BUG/MINOR: mux-h1/mux-fcgi: Sanitize TE header to only send "trailers"

Only chunk-encoded response payloads are supported by HAProxy. All other
transfer encodings are not supported and will be an issue if the HTTP
compression is enabled. So be sure only "trailers" is send in TE request
headers.

The patch is related to the issue #1301. It must be backported to all stable
versions. Be carefull for 2.0 and lower because the HTTP legacy must also be
fixed.

(cherry picked from commit f56e8465f067c84b820dbedd89e6f44f1e02c179)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>
(cherry picked from commit f9bb8d0fbcc945e15e8d1dc8ed792b27f1ca693b)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>