title: how to add sorted elements in the filter dropdown in activeadmin rails
body: <p>I have a Post model, PostSource model. A PostSource has many posts, a post belong to one PostSource.</p>
  
  <p>Using ActiveAdmin, in the Index action of the Post, I am displaying a filter of PostSource this way:</p>
  
  <pre><code>filter :post_source, label: 'Source'
  filter :category, as: :select, collection: Category.order(:name).collect { |cat| [cat.name, cat.id] }
  </code></pre>
  
  <p>with the controller as:</p>
  
  <pre><code>controller do
    def scoped_collection
      end_of_association_chain.includes(:post_source)
    end
  end
  </code></pre>
  
  <p>It displays the source, but does not display in a sorted order. How to sort the filter in this case?</p>
  
  <p>I tried adding sortable, order on filter but it does not seem to work</p>