title: Symfony form - Choicetype - Error "Array to string covnersion"
body: <p>I'm running Symfony 3.4 with FosUserBundle.
  I customized the registration form in order to get the <code>Roles</code> included :</p>
  
  <pre><code>&lt;?php
  
      $form-&gt;add('roles', ChoiceType::class, array(
          'choices' =&gt; array(
              'user' =&gt; 'ROLE_USER',
              'admin' =&gt; 'ROLE_ADMIN'
          ),
          'label' =&gt; 'Role :',
          'expanded' =&gt; true,
          'multiple' =&gt; true
      ));
  </code></pre>
  
  <p>Then roles are now selectable in the registration form through 2 new checkbox... but I'd like to display it with a <code>&lt;select&gt;</code> tag. When I set the <code>expanded</code> option to <code>false</code> I get a select, but if I set the <code>multiple</code> option to FALSE I get an error :</p>
  
  <blockquote>
    <p>Array to string conversion</p>
  </blockquote>
  
  <p>Any idea to fix my issue ?</p>
  
  <p>EDIT : below is the stack trace :</p>
  
  <pre><code>Symfony\Component\Debug\Exception\ContextErrorException:
  Notice: Array to string conversion
  
    at vendor\symfony\symfony\src\Symfony\Component\Form\ChoiceList\ArrayChoiceList.php:73
    at Symfony\Component\Form\ChoiceList\ArrayChoiceList-&gt;Symfony\Component\Form\ChoiceList\{closure}(array('ROLE_USER'))
    at call_user_func(object(Closure), array('ROLE_USER'))
       (vendor\symfony\symfony\src\Symfony\Component\Form\ChoiceList\ArrayChoiceList.php:158)
    at Symfony\Component\Form\ChoiceList\ArrayChoiceList-&gt;getValuesForChoices(array(array('ROLE_USER')))
       (vendor\symfony\symfony\src\Symfony\Component\Form\Extension\Core\DataTransformer\ChoiceToValueTransformer.php:32)
    at Symfony\Component\Form\Extension\Core\DataTransformer\ChoiceToValueTransformer-&gt;transform(array('ROLE_USER'))
       (vendor\symfony\symfony\src\Symfony\Component\Form\Form.php:1104)
    at Symfony\Component\Form\Form-&gt;normToView(array('ROLE_USER'))
       (vendor\symfony\symfony\src\Symfony\Component\Form\Form.php:350)
    at Symfony\Component\Form\Form-&gt;setData(array('ROLE_USER'))
       (vendor\symfony\symfony\src\Symfony\Component\Form\Extension\Core\DataMapper\PropertyPathMapper.php:49)
    at Symfony\Component\Form\Extension\Core\DataMapper\PropertyPathMapper-&gt;mapDataToForms(object(User), object(RecursiveIteratorIterator))
       (vendor\symfony\symfony\src\Symfony\Component\Form\Form.php:383)
    at Symfony\Component\Form\Form-&gt;setData(object(User))
       (vendor\friendsofsymfony\user-bundle\Controller\RegistrationController.php:70)
    at FOS\UserBundle\Controller\RegistrationController-&gt;registerAction(object(Request))
    at call_user_func_array(array(object(RegistrationController), 'registerAction'), array(object(Request)))
       (var\cache\dev\classes.php:4659)
    at Symfony\Component\HttpKernel\HttpKernel-&gt;handleRaw(object(Request), 1)
       (var\cache\dev\classes.php:4614)
    at Symfony\Component\HttpKernel\HttpKernel-&gt;handle(object(Request), 1, true)
       (vendor\symfony\symfony\src\Symfony\Component\HttpKernel\Kernel.php:200)
    at Symfony\Component\HttpKernel\Kernel-&gt;handle(object(Request))
       (web\app_dev.php:29)
  </code></pre>