title: Yii2 hiddenInput does not react on options value
body: <p>Going to change image field in model by changing value in hidden field:</p>
  
  <p>View:</p>
  
  <pre><code>&lt;?= HTML::activeHiddenInput($model, 'image_clean', [
      'id' =&gt; 'cleaner',
      'name' =&gt; 'cleaner',
      'value' =&gt; false
  ])
  ?&gt;
  &lt;?=
  Html::button('Remove logo', [
      'id' =&gt; 'btn_clean',
  ])
  ?&gt;
  </code></pre>
  
  <p>at the end of view:</p>
  
  <pre><code>    &lt;?php
      $this-&gt;registerJs(&lt;&lt;&lt;JS
      $('#btn_clean').on('click', function() {
          alert('Going to remove logo'); // Reachable!
          $('#cleaner').val(true);
      });
  JS
      );
      ?&gt;
  </code></pre>
  
  <p>Model:</p>
  
  <pre><code>public $image_clean;    // Remove logo
  
  public function rules() {
      return [
          //...
          [['image_clean'], 'boolean'],
          //...
      ];
  }
  
  public function attributeLabels() {
      return [
          //...
          'image_clean' =&gt; 'Remove logo',
          //...
      ];
  }
  
  public function beforeValidate() {
      if($this-&gt;image_clean) { }  // Never!
      return parent::beforeValidate();
  }
  
  public function beforeSave($insert) {
      if($this-&gt;image_clean) { } // Never!
      return parent::beforeSave($insert);
  }
  </code></pre>
  
  <p>Unfortunately, $this->image_clean in model's beforeValidate / beforeSave always false. Why?</p>
  
  <p>Javascript btn_clean handler works as it should be.</p>