title: Propel form type model w. group_by is rendered without property display
body: <p>Env: Symfony2 2.7 / Propel 1.6</p>
  
  <p>I've created a choice form type like that:</p>
  
  <pre><code>    $builder-&gt;add('mychoice', 'model', array(
              'class' =&gt; 'Foo\\Bar',
              'query' =&gt; FooBarQuery::create()-&gt;filterBySomething(true),
              'group_by' =&gt; 'example',
              'property' =&gt; 'title',
              'multiple' =&gt; false,
              'expanded' =&gt; false,
          ));
  </code></pre>
  
  <p>The rendering choice list is ok with good optgroup select options but the title's property is not displayed - id's property instead. If I remove the group_by option, the title property is well displayed.</p>
  
  <p>What's wrong?</p>