BUG/MINOR: mux-h1: Always set the session on frontend h1 stream

BUG/MINOR: mux-h1: Always set the session on frontend h1 stream

The session is always defined for a frontend connection. When a new client
connection is established, the session is set for the first H1 stream. But on
keep-alived connections, it is not set for the followings H1 streams while it is
possible.

This patch is tagged as a bug because it fixes an inconsistency in the H1
streams creation. But it does not fixed a known bug.

This patch must be backported as far as 2.0.