BUG/MEDIUM: mux-h2: make sure we don't emit TE headers with anything but "trailers"

BUG/MEDIUM: mux-h2: make sure we don't emit TE headers with anything but "trailers"

While the H2 parser properly checks for the absence of anything but
"trailers" in the TE header field, we forget to check this when sending
the request to an H2 server. The problem is that an H2->H2 conversion
may keep "gzip" and fail on the next stage.

This patch makes sure that we only send "TE: trailers" if the TE header
contains the "trailers" token, otherwise it's dropped.

This fixes issue #464 and should be backported till 1.9.