MINOR: stats: send content-length with the redirect to allow keep-alive

After a POST on the stats admin page, a 303 is emitted. Unfortunately
this 303 doesn't contain a content-length, which forces the connection
to be closed and reopened. Let's simply add a content-length: 0 to solve
this.

src/dumpstats.c

@@ -5176,6 +5176,7 @@ static int stats_send_http_redirect(struct stream_interface *si)
 		     "Content-Type: text/plain\r\n"
 		     "Connection: close\r\n"
 		     "Location: %s;st=%s%s%s%s\r\n"
+		     "Content-length: 0\r\n"
 		     "\r\n",
 		     uri->uri_prefix,
 		     ((appctx->ctx.stats.st_code > STAT_STATUS_INIT) &&