[4.x] Add Vary: X-Requested-With header to CP responses (#10016)

Co-authored-by: duncanmcclean <duncanmcclean@users.noreply.github.com>

src/Http/Middleware/CP/AddVaryHeaderToResponse.php

@@ -0,0 +1,17 @@
+<?php
+
+namespace Statamic\Http\Middleware\CP;
+
+use Closure;
+
+class AddVaryHeaderToResponse
+{
+    public function handle($request, Closure $next)
+    {
+        $response = $next($request);
+
+        $response->headers->set('Vary', 'X-Requested-With');
+
+        return $response;
+    }
+}

src/Providers/CpServiceProvider.php

@@ -92,6 +92,7 @@ protected function registerMiddlewareGroups()
             \Statamic\Http\Middleware\CP\BootPreferences::class,
             \Statamic\Http\Middleware\CP\BootUtilities::class,
             \Statamic\Http\Middleware\CP\CountUsers::class,
+            \Statamic\Http\Middleware\CP\AddVaryHeaderToResponse::class,
             \Statamic\Http\Middleware\DeleteTemporaryFileUploads::class,
         ]);
     }