title: Validate checkbox not mapped to entity in a symfony2 form
body: <p>I add a non mapped field to a symfony2 form type:</p>
  
  <pre><code>$builder-&gt;add('terms','checkbox', array('mapped' =&gt; false,
          'constraints' =&gt; array(new NotBlank())));
  </code></pre>
  
  <p>But the NotBlank() constraint is not working! Only if I change the type from 'checkbox' to 'text' it is working as expected. So how can I validate a checkbox? Of course I tried with 'True()', 'EqualTo()' and 'Length(...)' constraints too. But without success. I also tried different POST values (1/0, true/false, on/off...) for the field. </p>
  
  <p>What is the big difference between a checkbox field and a text field regarding form field validation in symfony2?</p>
  
  <p>Thanx
  Stef</p>