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

Tests/FilesystemTest.php

@@ -797,7 +797,7 @@ public function testSymlink()
         $file = $this->workspace.\DIRECTORY_SEPARATOR.'file';
         $link = $this->workspace.\DIRECTORY_SEPARATOR.'link';
 
-        // $file does not exists right now: creating "broken" links is a wanted feature
+        // $file does not exist right now: creating "broken" links is a wanted feature
         $this->filesystem->symlink($file, $link);
 
         $this->assertTrue(is_link($link));