BUG/MINOR: mux-h1: Skip trailers for non-chunked outgoing messages

BUG/MINOR: mux-h1: Skip trailers for non-chunked outgoing messages

Unlike H1, H2 messages may contains trailers while the header "Content-Length"
is set. Indeed, because of the framed structure of HTTP/2, it is no longer
necessary to use the chunked transfer encoding. So Trailing HEADERS frames,
after all DATA frames, may be added on messages with an explicit content length.

But in H1, it is impossible to have trailers on non-chunked messages. So when
outgoing messages are formatted by the H1 multiplexer, if the message is not
chunked, all trailers must be dropped.

This patch must be backported to 2.0 and 1.9. However, the patch will have to be
adapted for the 1.9.