BUG/MINOR: h2: properly set the direction flag on HTX response

BUG/MINOR: h2: properly set the direction flag on HTX response

In 1.9-dev, a new flag was introduced on the start line with commit
f1ba18d7b ("MEDIUM: htx: Don't rely on h1_sl anymore except during H1
header parsing") to designate a response message: HTX_SL_F_IS_RESP.

Unfortunately as it was done in parallel to the mux_h2 support for
the backend, it was never integrated there. It was not used by then
so this remained unnoticed for a while.

However the http_client now uses it, and missing that flag prevents
it from using the H2 mux, so let's properly add it.

There's no point in backporting this far away, but since the http_client
is fully operational in 2.6 it would make sense to backport this fix at
least there to secure the code.