title: Active admin: Custom check_box input
body: <p>I need to show a dynamic list of values to show on an Active Admin edit screen as checkboxes, where the list comes from code (not database).  I can do that pretty easily, but I can't figure out how to show some of those as being checked.  <a href="https://i.stack.imgur.com/khJRz.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/khJRz.png" alt="checkboxes"></a></p>
  
  <p>Here's a simplified example of what I'm trying to do:</p>
  
  <pre><code>names = %w(Sam Darcy Ernie)
  pairs = Hash[names.zip(names)]
  f.input :buddies, as: :check_boxes, collection: pairs, checked: %w(Sam)
  </code></pre>
  
  <p>What I was hoping for is to show the 3 checkboxes and have the "Sam" box checked.  None are checked though.  What can I do to control which checkboxes are checked?</p>