title: Use choice_value option in ChoiceType (Symfony 3)
body: <p>I use Symfony 3.0.4. I wish replace the value to increment by my label.
  I referred to the official documentation unsuccessfully.
  I also tried <a href="https://symfony.com/doc/master/reference/forms/types/choice.html#choices-as-values" rel="nofollow">choices_as_values</a> unsuccessfully too :(</p>
  
  <p>This is a piece of my buildForm() function :</p>
  
  <pre><code>-&gt;add(
      'fieldName',
      ChoiceType::class,
      array(
          'choices' =&gt; array(
              'Yes' =&gt; true,
              'No' =&gt; true,
              'Maybe' =&gt; true,
          ),
          'choice_value' =&gt; function ($choice) {
              return $choice;
          },
      )
  )
  </code></pre>
  
  <p>And I have a return error 500 : </p>
  
  <blockquote>
    <p>Notice: Array to string conversion</p>
  </blockquote>
  
  <p>Do you have any idea? Thanks a lot</p>