Merge branch '5.4' into 6.0

* 5.4:
  fix merge
  do not use mocks in tests when not necessary
  [Mailer] Missing import in first example
  [Security] Add test case for user not found
  [SecurityBundle] Use config's secret in remember-me signatures

Author: xabbuh

README.md

@@ -13,6 +13,7 @@ $ composer require symfony/mailer
 ```php
 use Symfony\Component\Mailer\Transport;
 use Symfony\Component\Mailer\Mailer;
+use Symfony\Component\Mime\Email;
 
 $transport = Transport::fromDsn('smtp://localhost');
 $mailer = new Mailer($transport);