title: yii2 exist validator with scenario does not work
body: <p>Scenario "withAppId" was defined and rule for "application_id" was linked to one, an ActiveField was added to form, but model doesn't save the field. What I did wrong?</p>
  
  <p>Controller:</p>
  
  <pre><code>public function actionIndex()
  {
      $post = Yii::$app-&gt;request-&gt;post();
      $model = new Option();
      $model-&gt;scenario = 'withAppId';
      if ($model-&gt;load($post) &amp;&amp; $model-&gt;save()) {
          Yii::$app-&gt;session-&gt;addFlash('success', 'Запись успешно сохранена.');
          return $this-&gt;redirect(['index']);
      } else {
          //form rendering
      }
  }
  </code></pre>
  
  <p>Model:</p>
  
  <pre><code>public function rules()
  {
      return [
          ['key', 'trim'],
          ['key', 'required'],
          ['key', 'string', 'max' =&gt; 100],
  
          ['option', 'string', 'max' =&gt; 255],
  
          ['type', 'required'],
          ['type', 'in', 'range' =&gt; array_keys(self::$TYPES)],
  
          ['uploaded', 'image', 
              'extensions'    =&gt; 'png, jpg, jpeg, gif',
              'maxWidth'      =&gt; 1000,
              'maxHeight'     =&gt; 1000,
              'maxSize'       =&gt; 1024*1024,
          ],
  
          ['application_id', 'exist',
              'targetClass' =&gt; Application::className(),
              'on'          =&gt; ['withAppId'],
          ],
      ];
  }
  
  public function scenarios()
  {
      $cenarios = parent::scenarios();
      $cenarios['withAppId'] = $cenarios[parent::SCENARIO_DEFAULT];
      return $cenarios;
  }
  </code></pre>
  
  <p>When <code>'on' =&gt; ['withAppId'],</code> is removed the saving works properly.</p>