BUG/MINOR: mux-h1: Add the header connection in lower case in outgoing messages

When necessary, this header is directly added in outgoing messages by the H1
multiplexer. Because there is no HTX conversion first, the header name is not
converserted to its lower case version. So, it must be added in lower case by
the multiplexer.

This patch must be backported to 2.0 and 1.9.
diff --git a/src/mux_h1.c b/src/mux_h1.c
index 317f1a5..21deb35 100644
--- a/src/mux_h1.c
+++ b/src/mux_h1.c
@@ -1642,7 +1642,7 @@
 					/* There is no "Connection:" header and
 					 * it the conn_mode must be
 					 * processed. So do it */
-					n = ist("Connection");
+					n = ist("connection");
 					v = ist("");
 					h1_process_output_conn_mode(h1s, h1m, &v);
 					if (v.len) {