Merge branch '3.4' into 4.3

* 3.4:
  [Filesystem] [Serializer] fixes English grammar typo
  [Monolog Bridge] Fixed accessing static property as non static.
  Improve Symfony description
  [Validator] Add Japanese translation
  Remove some unused methods parameters
  Avoid empty \"If-Modified-Since\" header in validation request
  [Validator] ConstraintValidatorTestCase: add missing return value to mocked validate method calls

Author: fabpot

HttpCache/HttpCache.php

@@ -382,7 +382,9 @@ protected function validate(Request $request, Response $entry, $catch = false)
         }
 
         // add our cached last-modified validator
-        $subRequest->headers->set('if_modified_since', $entry->headers->get('Last-Modified'));
+        if ($entry->headers->has('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

Tests/HttpCache/HttpCacheTest.php

@@ -859,6 +859,7 @@ public function testValidatesCachedResponsesUseSameHttpMethod()
     public function testValidatesCachedResponsesWithETagAndNoFreshnessInformation()
     {
         $this->setNextResponse(200, [], 'Hello World', function ($request, $response) {
+            $this->assertFalse($request->headers->has('If-Modified-Since'));
             $response->headers->set('Cache-Control', 'public');
             $response->headers->set('ETag', '"12345"');
             if ($response->getETag() == $request->headers->get('IF_NONE_MATCH')) {