Correct use of the message manager in the loadMessageData fixture class

When launching raw_install I get this error:
 > PHP Catchable fatal error:  Argument 1 passed to Claroline\CoreBundle\Manager\MessageManager::send() must be an instance of Claroline\CoreBundle\Entity\Message, instance of Claroline\CoreBundle\Entity\User given, called in /home/maxime/workspace/Claroline/vendor/claroline/core-bundle/Claroline/CoreBundle/DataFixtures/Optional/LoadMessagesData.php on line 61 and defined in /home/maxime/workspace/Claroline/vendor/claroline/core-bundle/Claroline/CoreBundle/Manager/MessageManager.php on line 101

So here is the patch for this error.