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.
diff --git a/src/mux_h1.c b/src/mux_h1.c
index 9091159..d4c109f 100644
--- a/src/mux_h1.c
+++ b/src/mux_h1.c
@@ -571,7 +571,7 @@
 
 		/* For frontend connections we should always have a session */
 		if (!sess)
-			sess = h1c->conn->owner;
+			h1s->sess = sess = h1c->conn->owner;
 
 		/* Timers for subsequent sessions on the same HTTP 1.x connection
 		 * measure from `now`, not from the connection accept time */