system test: add caching header

README.md

@@ -128,6 +128,8 @@ you need to first build and configure nginx.  Set it up something like:
         root /path/to/mod_pagespeed/src/install;
         index index.html;
 
+        add_header Cache-Control "public, max-age=600";
+
         location /mod_pagespeed_test/no_cache/ {
           add_header Cache-Control no-cache;
         }