Do not cache 404 [#3025]

system/pages/notfound.md

@@ -2,4 +2,5 @@
 title: Not Found
 routable: false
 notfound: true
+expires: 0
 ---

system/src/Grav/Common/Grav.php

@@ -261,7 +261,7 @@ class Grav extends Container
         );
 
         $default = function (ServerRequestInterface $request) {
-            return new Response(404);
+            return new Response(404, ['Expires' => 0, 'Cache-Control' => 'no-cache, no-store, must-revalidate'], 'Not Found');
         };
 
         /** @var Debugger $debugger */