title: Yii2 DateRangePicker default value does not work
body: <p>I am using <a href="https://github.com/kartik-v/yii2-date-range" rel="nofollow">kartik date-range</a> in order to select date range in my yii2 app, but after trying some ways i could not set default value to date range picker, please see my codes :   </p>
  
  <pre><code>&lt;?php  echo $form-&gt;field($model, 'date')-&gt;widget(DateRangePicker::className(), [
      'value'=&gt; '2016-05-17 00:00:00',
      'convertFormat'=&gt;true,
      'pluginOptions'=&gt;[
          'timePicker'=&gt;true,
          'timePickerIncrement'=&gt;15,
          'locale'=&gt;['format'=&gt;'Y-m-d H:i:s']
      ]
  ])
  ?&gt;
  </code></pre>
  
  <p>date input box does not show default value and it is empty </p>