title: Form datetime type validation error
body: <p>I'm using collection form type, where children has datetime type input as single text. But when I submit data in validation fails and getErrorsAsString shows this strange error:</p>
  
  <pre><code>inputDateTime:
          ERROR: This value is not valid.
          date:
              No errors
          time:
              No errors
  </code></pre>
  
  <p>Here is how I add my field</p>
  
  <pre><code>//form child
  $builder-&gt;add('inputDateTime', 'datetime', array(
      'date_widget' =&gt; 'single_text',
      'time_widget' =&gt; 'single_text'
  ));
  </code></pre>
  
  <p>Why inputDateTime has errors but neither date or time has errors? My locale is set and I tried to add date format but it didn't help.</p>