title: Symfony2 Forms BooleanToStringTransformer Issue
body: <p>I have a boolean field that I've put in a form as a choice field (yes or no).
  I would get 0 or 1 without data transformer.
  I added a view BooleanToStringTransformer (which seemed reasonable) :</p>
  
  <pre><code>$builder
          -&gt;add(
              $builder-&gt;create('myBooleanField', 'choice', array(
                  'choices' =&gt; array(true =&gt; 'Yes', false =&gt; 'No'),
              ))
              -&gt;addViewTransformer(new BooleanToStringTransformer('1'))
          )
  </code></pre>
  
  <p>And when I try to display the form, I get the error "Expected a Boolean.".
  My field is set to false before creating the form though.</p>
  
  <p>I tried to set it as a model transformer: the form is dispayed, but when I submit it my field is declared invalid.</p>
  
  <p>What am I doing wrong?</p>
  
  <p>Edit: I nearly got it now.</p>
  
  <ul>
  <li>I used a model transformer instead of a view transformer (the choice field works with strings or integers, not booleans)</li>
  <li>I changed my choice list from <code>array(true =&gt; 'Yes', false =&gt; 'No')</code> to <code>array('yes' =&gt; 'Yes', 'no' =&gt; 'No')</code></li>
  </ul>
  
  <p>So the code now looks like <code>-&gt;addModelTransformer(new BooleanToStringTransformer('yes'))</code></p>
  
  <p>Data transformation works, except that my field is always set to true, whatever value I choose.</p>
  
  <p>What's wrong?</p>