[HttpCache] fixed if-modified-since header default to an empty string…

… if the last-modified header was not set

src/Symfony/Component/HttpKernel/HttpCache/HttpCache.php

@@ -5,10 +5,6 @@
  *
  * (c) Fabien Potencier <fabien@symfony.com>
  *
- * This code is partially based on the Rack-Cache library by Ryan Tomayko,
- * which is released under the MIT license.
- * (based on commit 02d2b48d75bcb63cf1c0c7149c077ad256542801)
- *
  * For the full copyright and license information, please view the LICENSE
  * file that was distributed with this source code.
  */
@@ -352,7 +348,7 @@ protected function validate(Request $request, Response $entry, $catch = false)
         }
 
         // add our cached last-modified validator
-        $subRequest->headers->set('if_modified_since', $entry->headers->get('Last-Modified'));
+        $subRequest->headers->set('if_modified_since', $entry->headers->get('Last-Modified', ''));
 
         // Add our cached etag validator to the environment.
         // We keep the etags from the client to handle the case when the client