BUG/MEDIUM: sample: http_date() doesn't provide the right day of the week

Gregor Kovač reported that http_date() did not return the right day of the
week. For example "Sat, 22 Jan 2016 17:43:38 GMT" instead of "Fri, 22 Jan
2016 17:43:38 GMT". Indeed, gmtime() returns a 'struct tm' result, where
tm_wday begins on Sunday, whereas the code assumed it began on Monday.

This patch must be backported to haproxy 1.5 and 1.6.
diff --git a/src/proto_http.c b/src/proto_http.c
index e362a96..2f76afe 100644
--- a/src/proto_http.c
+++ b/src/proto_http.c
@@ -11973,7 +11973,7 @@
  */
 static int sample_conv_http_date(const struct arg *args, struct sample *smp, void *private)
 {
-	const char day[7][4] = { "Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun" };
+	const char day[7][4] = { "Sun", "Mon", "Tue", "Wed", "Thu", "Fri", "Sat" };
 	const char mon[12][4] = { "Jan", "Feb", "Mar", "Apr", "May", "Jun", "Jul", "Aug", "Sep", "Oct", "Nov", "Dec" };
 	struct chunk *temp;
 	struct tm *tm;