title: Rails mass-assign for admin users
body: <p>I'm using rolify to manage user roles and when I try to update the roles via a user form I receive this:</p>
  
  <pre><code>can't mass-assign protected attributes: role_ids
  </code></pre>
  
  <p>This is comforting, but it has me wondering, how do I then allow admin to update user roles via mass-assign yet not allow normal users to?</p>