BUG/MEDIUM: http: don't enable auto-close on the response side

There is a bug where "option http-keep-alive" doesn't force a response
to stay in keep-alive if the server sends the FIN along with the response
on the second or subsequent response. The reason is that the auto-close
was forced enabled when recycling the HTTP transaction and it's never
disabled along the response processing chain before the SHUTR gets a
chance to be forwarded to the client side. The MSG_DONE state of the
HTTP response properly disables it but too late.

There's no more reason for enabling auto-close here, because either it
doesn't matter in non-keep-alive modes because the connection is closed,
or it is automatically enabled by process_stream() when it sees there's
no analyser on the stream.

This bug also affects 1.5 so a backport is desired.
diff --git a/src/proto_http.c b/src/proto_http.c
index 77a5207..67e482f 100644
--- a/src/proto_http.c
+++ b/src/proto_http.c
@@ -5272,11 +5272,13 @@
 			s->res.flags |= CF_EXPECT_MORE;
 	}
 
-	/* we're removing the analysers, we MUST re-enable events detection */
+	/* we're removing the analysers, we MUST re-enable events detection.
+	 * We don't enable close on the response channel since it's either
+	 * already closed, or in keep-alive with an idle connection handler.
+	 */
 	channel_auto_read(&s->req);
 	channel_auto_close(&s->req);
 	channel_auto_read(&s->res);
-	channel_auto_close(&s->res);
 
 	/* we're in keep-alive with an idle connection, monitor it if not already done */
 	if (srv_conn && LIST_ISEMPTY(&srv_conn->list)) {