title: Filter activeadmin with hstore
body: <p>I would like to use activeadmin filters with hstore:</p>
  
  <p>In model I have got column amenities with Room.</p>
  
  <p>I would like to do sth like this:</p>
  
  <blockquote>
    <p>filter :amenities, :as => :select, :collection => proc
    {Room.all.map{|r| r.amenities} }</p>
  </blockquote>