title: Set the nil value for session_id while session timeout - devise
body: <p>In rails 3.2.13 project, I am using devise(1.4.2) plugin for authentication system. Here, I need to update current_user's session_id to nil value after session timeout. How to override the timeout function?</p>
  
  <p>After sign in, session_id field will update like "A9nEDMZFl7D4ytSfWxrY". After sign out, session_id field will update with nil value. Now I need to update the same nil value for session expiry. How to modify session expiry function?</p>
  
  <p>In session's create action, I am updating session_id using Devise.friendly_token</p>