title: Assert\Type not working in Symfony2 form validation
body: <p>I have a form containing a field that expects an integer. This is the field definition in the form type entity definition:</p>
  
  <pre><code>/**
   * @ORM\Column(type="integer", nullable=true)
   * @Assert\Type(type="integer", message="Number of pieces must be a number.")
   * @Assert\GreaterThanOrEqual(value=1, message="Number of pieces cannot be lower than 1.")
   */
  protected $numberOfPiecesSent;
  </code></pre>
  
  <p>The relevant form builder looks like this:</p>
  
  <pre><code>public function buildForm(FormBuilderInterface $builder, array $options)
  {
      $builder
          -&gt;add('numberOfPiecesSent', 'integer', array('label' =&gt; 'Number of pieces sent:', 'required' =&gt; false));
  }
  </code></pre>
  
  <p>When I submit the form with a non-numerical value in the field (say, 'aaa'), it just saves the form and leaves the field numberOfPiecesSent NULL in the database instead of failing on validation. I also tried to make the field non-NULL, but that didn't help. Any ideas why this isn't working, please?</p>