title: Assert\Url does validates invalid urls in Symfony3
body: <p>I have been adding some constraints in my form/url, and when testing it, I realized that it would actually accept any types of string???</p>
  
  <p>Here is my code:</p>
  
  <p>
  
  <pre><code>namespace OC\PrepBundle\Entity;
  
  use Doctrine\ORM\Mapping as ORM;
  use Doctrine\Common\Collections\ArrayCollection;
  use Doctrine\ORM\EntityManager;
  use Symfony\Component\Validator\Constraints as Assert;
  
  
  /**
   * Video
   *
   * @ORM\Table(name="video")
   * @ORM\Entity(repositoryClass="OC\PrepBundle\Repository\VideoRepository")
   */
  class Video
  {
      /**
       * @var int
       *
       * @ORM\Column(name="id", type="integer")
       * @ORM\Id
       * @ORM\GeneratedValue(strategy="AUTO")
       */
      private $id;
  
      /**
       *
       * @ORM\Column(name="video_url", type="text")
       *
       * @Assert\UrL(
       *     message ="This url is not valid",
       *     checkDNS = true
       *     )
       *
       */
      private $videourl;
  </code></pre>
  
  <p>Do you guys have any idea?</p>
  
  <p>Thanks</p>