title: sess_expire_on_close not working
body: <p>I have a problem..in <code>$config['sess_expire_on_close']   = TRUE;</code>
  when i close the browser..the session still save in the database.
  and i can still see the data in <code>user_data</code> column. </p>
  
  <p><strong>config.php</strong></p>
  
  <pre><code>  $config['sess_cookie_name']     = 'ci_session';
    $config['sess_expiration']        = 2400;
     $config['sess_expire_on_close']  = TRUE;
    $config['sess_encrypt_cookie']    = FALSE;
    $config['sess_use_database']  = TRUE;
    $config['sess_table_name']    = 'ci_sessions';
    $config['sess_match_ip']  = FALSE;
    $config['sess_match_useragent']   = TRUE;
   $config['sess_time_to_update'] = 300;
  </code></pre>
  
  <p>Need help..
  the purpose is to show all online users. </p>
  
  <p>I'm using CI version 2.1.3</p>