title: Yii2, Set multiple Value Select2
body: <p>I have load <strong>select2</strong> data like this : </p>
  
  <pre><code>$data = ArrayHelper::map(ContactGroups::find()-&gt;where(['group_status'=&gt;'ACTIVE'])-&gt;asArray()-&gt;all(),'group_id', 'group_name'); 
  
  echo $form-&gt;field($model, 'group_id')-&gt;widget(Select2::classname(), [
   'data' =&gt; $data,
   'model' =&gt; $model,
    'language' =&gt; 'en',
    'options' =&gt; ['placeholder' =&gt; Yii::t('modules','Pilih Kelompok')],
    'pluginOptions' =&gt; [
      'allowClear' =&gt; true,
      'multiple' =&gt; true,
    ],
  ])-&gt;label('Kelompok');
  </code></pre>
  
  <p><code>$data</code> variable returning result :</p>
  
  <pre><code>Array
  (
      [1] =&gt; Tanpa Kategori
      [3] =&gt; Bisnis
      [4] =&gt; Kawan
      [5] =&gt; Bisnis Kerang
      [6] =&gt; Bisnis Selang
      [99] =&gt; Keluarga
  )
  </code></pre>
  
  <p>and <strong>select2</strong> working properly, but I can't show selected value or <code>initial value</code>. is I've missed something ?</p>