title: Symfony2 choice (checkbox) in form always false
body: <p>I have problem with Symfony2 form choice field (checkbox). It is <strong>always false</strong> after submit even if I check it to true.</p>
  
  <p>Entity class:</p>
  
  <pre><code>/**
   * @var boolean
   *
   * @ORM\Column(name="isActive", type="boolean")
   */
  private $isActive;
  </code></pre>
  
  <p>Form builder class:</p>
  
  <pre><code>...-&gt;add('isActive')
  </code></pre>
  
  <p>And var_dump when I submit form with field unchecked or checked, result is still the same:</p>
  
  <pre><code>private 'isActive' =&gt; boolean false
  </code></pre>
  
  <p><strong>When I change the form builder class like this:</strong></p>
  
  <pre><code>...-&gt;add('isActive', 'choice', ['choices' =&gt; [true =&gt; 'Yes', false =&gt; 'No']])
  </code></pre>
  
  <p>it works then with integer value:</p>
  
  <pre><code>private 'isActive' =&gt; int 1
  </code></pre>
  
  <p>Does anybody know what am I doing bad?</p>