Array to string conversion exception on DateField placeholder attr
<p>I have a form with a DateField like this:</p>

<pre><code>public function buildForm(FormBuilderInterface $builder, array $options)
{
    $builder
        -&gt;add('started', 'date', [
            'label' =&gt; 'Début',
            'placeholder' =&gt; 'ex: 01/09/2007',
            'widget' =&gt; 'single_text',
            'format' =&gt; 'dd/MM/yyyy',
            'attr' =&gt; [
               'autocomplete' =&gt; 'off',
             ],
        ]);
}
</code></pre>

<p>And when I want to view it in a page, I have an exception occurs:</p>

<blockquote>
  <p>An exception has been thrown during the rendering of a template ("Notice: Array to string conversion") in form_div_layout.html.twig at line 322.</p>
</blockquote>

<p>In logs, I have this details:</p>

<blockquote>
  <p>at Twig_Template ->displayBlock ('widget_attributes', array('value' => object(ExperienceWork), 'attr' => array('autocomplete' => 'off', 'placeholder' => array('year' => 'ex: 01/09/2007', 'month' => 'ex: 01/09/2007', 'day' => 'ex: 01/09/2007'),[...]</p>
</blockquote>

<p>I think Twig can not resolve array placeholder, but in <a href="https://github.com/symfony/Form/blob/master/Extension/Core/Type/DateType.php" rel="nofollow">DateType.php source</a> I found that this behaviour can not be avoid. I think I make somethings wrong, but I don't know what.</p>