title: Get a string that represents a user's CanCan abilities
body: <p>I want to cache a <code>Post</code> view, but the view depends on the permissions of the current user (e.g., I only show the "edit" link if <code>current_user.can?(:edit, @post)</code>)</p>
  
  <p>So I'd like my cache key to include a representation of the current user's CanCan abilities, so that I can invalidate the cache when the user's abilities change</p>
  
  <p>SO: how can I get a string that represents the current user's abilities such that 2 different users with the same abilities will generate the same "ability string"?</p>
  
  <p>I've tried <code>user.ability.inspect</code>, but this doesn't produce the same string for different users who have the same abilities</p>