title: datepicker in filters portion not show with created_at and updated_at fields using Active Admin Rails
body: <p>I'm using Active admin and datepicker in filters portion(index page) not show with created_at and updated_at fields. I search enough and not find solution in any place. </p>
  
  <pre><code>   filter :created_at ,:as =&gt; :date_picker   #not working
  </code></pre>
  
  <p>How can i do this? </p>