title: How to set to checked by default a filter_checkbox choice field with symfony2.0?
body: <p>The title says pretty much everything, but here are pieces of my code to illustrate what I'm trying to do.</p>
  
  <p>Here is the Form Type</p>
  
  <pre class="lang-php prettyprint-override"><code>//[...]
  class ActivityFilterType extends AbstractType
  {
    //[...]
    public function buildForm(FormBuilder $builder, array $options)
    {
          $builder-&gt;add('filter_checkbox', 'choice', array(
                  'label'    =&gt; $this-&gt;translator-&gt;trans('form.label.sexFilter'),
                  'choices'  =&gt; array(1 =&gt; ucfirst($this-&gt;translator-&gt;transChoice('sex.female', 2)), 2 =&gt; ucfirst($this-&gt;translator-&gt;transChoice('sex.male', 2))),
                  'multiple' =&gt; true,
                  'expanded' =&gt; true,
                  'required' =&gt; false,
                  'empty_value' =&gt; false,
                  /*
                     // those ones didn't make it
                     'attr'     =&gt; array( 1=&gt;array('checked' =&gt; 'checked'), 2=&gt;array('checked' =&gt; 'checked') ),
                     'preferred_choices' =&gt; array(1, 2),
                  */
          ));
    }
    //[...]
  }
  </code></pre>
  
  <p>And the form Template</p>
  
  <pre class="lang-html prettyprint-override"><code>&lt;div class="filter-message"&gt;
      &lt;div class="select-group"&gt;
          {{ form_label(form.filter_dropdown) }}
          &lt;div class="input control-group filter" &gt;
              {{ form_widget(form.filter_dropdown) }}
          &lt;/div&gt;
      &lt;/div&gt;
      &lt;div class="checkbox"&gt;
          {{ form_label(form.filter_checkbox) }}
          {{ form_widget(form.filter_checkbox, {'attr':{'checked':checked}}) }} 
          &lt;!-- that didn't do it neither --&gt;
      &lt;/div&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>Thanks</p>