title: How do I set default index sort_order that ignore case
body: <p>Current I am following activeadmin guide on <a href="https://activeadmin.info/3-index-pages.html#index-default-sort-order" rel="nofollow noreferrer">their site</a>:  </p>
  
  <pre><code>ActiveAdmin.register Brand do
    config.sort_order = 'brand_name_asc'
  end
  </code></pre>
  
  <p>But turn out this method sorts the selected column (brand_name) alphabetically case sensitive.  </p>
  
  <p>How can I make a default sort_order that ignore case?</p>