title: Symfony set value checked on a form type choice
body: <p>I create a form with FormBuilder with Symfony like :</p>
  
  <pre><code>$builder
              -&gt;add('timeBarOpen', 'time', array('label' =&gt; 'Ouverture Bar', 'attr' =&gt; array('class' =&gt; 'form-control')))
              -&gt;add('timeBarClose', 'time', array('label' =&gt; 'Fermeture Bar', 'attr' =&gt; array('class' =&gt; 'form-control')))
              -&gt;add('timeStartHappyHour', 'time', array('label' =&gt; 'Début Happy Hour *', 'attr' =&gt; array('class' =&gt; 'form-control')))
              -&gt;add('timeEndHappyHour', 'time', array('label' =&gt; 'Fin Happy Hour *', 'attr' =&gt; array('class' =&gt; 'form-control')))
              -&gt;add('day', 'choice', [
                  'choices' =&gt; $days,
                  'multiple' =&gt; true,
                  'expanded' =&gt; true,
                  'label' =&gt; 'Jour(s) *',
              ])
          ;
  </code></pre>
  
  <p>$days is an array :</p>
  
  <pre><code>$days = array(
              'Monday'    =&gt; 'Lundi',
              'Tuesday'   =&gt; 'Mardi',
              'Wednesday' =&gt; 'Mercredi',
              'Thursday'  =&gt; 'Jeudi',
              'Friday'    =&gt; 'Vendredi',
              'Saturday'  =&gt; 'Samedi',
              'Sunday'    =&gt; 'Dimanche',
          );
  </code></pre>
  
  <p>So, this field type "choice" generates multiple checkboxes and I need them <em>all</em> to be checked by defaut when the form is created.</p>
  
  <p>How can I do that?</p>