title: Yii2 whenClient validation issue
body: <p>For some reason I cannot get conditional rule 'required' to work. Even if I reduce the condition to "always return false", required-validation seems to check this unnecessary field:</p>
  
  <pre class="lang-php prettyprint-override"><code>public function rules() {
      return [
          [['order_id', 'product_id', 'quantity'], 'required'],
  
          ['product_date', 'required', 
           'whenClient' =&gt; "function(attribute, value) {
                                return false;
                            }"
          ],  
  
         // more rules here
          [['date_create', 'date_update', 'product_date'], 'safe'],
          // more rules here
      ];
  }
  </code></pre>
  
  <p>On form submit save() fails and $model->getErrors() points to product_date as a necessary field. What have I missed? Thank you in advance.</p>