title: Yii2 DateTime model validation format issue
body: <p>Created a model with a dateTime field called 'until', the name is not important just that it is a date time.
  Here is it's validation in the rules of the model.</p>
  <pre><code>['until', 'datetime', 'format' =&gt; 'Y-m-d H:i:s'],
  </code></pre>
  <p>Later after I instantiate the model and update the field I get a validation error.</p>
  <pre><code>$model-&gt;until = '2017-10-15 14:30:04';
  $eventRepeat-&gt;validate();
  $errors = $model-&gt;errors;
  </code></pre>
  <p>$errors['until'][0] will then be 'The format of Until is invalid.'</p>
  <p>I was not expecting this error.</p>