MINOR: proxy: Store http-send-name-header in lower case

All HTTP header names are now handled in lower case. So this one is now stored
in lower case. It will simplify some processing in HTTP muxes.
diff --git a/src/cfgparse-listen.c b/src/cfgparse-listen.c
index 656031b..a5d46f5 100644
--- a/src/cfgparse-listen.c
+++ b/src/cfgparse-listen.c
@@ -1396,10 +1396,11 @@
 			goto out;
 		}
 
-		/* set the desired header name */
+		/* set the desired header name, in lower case */
 		free(curproxy->server_id_hdr_name);
 		curproxy->server_id_hdr_name = strdup(args[1]);
 		curproxy->server_id_hdr_len  = strlen(curproxy->server_id_hdr_name);
+		ist2bin_lc(curproxy->server_id_hdr_name, ist2(curproxy->server_id_hdr_name, curproxy->server_id_hdr_len));
 	}
 	else if (!strcmp(args[0], "block")) {
 		ha_alert("parsing [%s:%d] : The '%s' directive is not supported anymore since HAProxy 2.1. Use 'http-request deny' which uses the exact same syntax.\n", file, linenum, args[0]);