BUG/MINOR: mux-h1: Add "transfer-encoding" header on outgoing requests if needed

BUG/MINOR: mux-h1: Add "transfer-encoding" header on outgoing requests if needed

As for outgoing response, if an HTTP/1.1 or above request is sent to a server
with neither the headers "content-length" nor "transfer-encoding", it is
considered as a chunked request and the header "transfer-encoding: chunked" is
automatically added. Of course, it is only true for requests with a
body. Concretely, it only happens for incoming HTTP/2 requests sent to an
HTTP/1.1 server.

This patch must be backported to 1.9.