Theme cannot add classes via preprocess to webform_advanced_redirection_form

Bugfix for [webform.pages.inc]

/**
 * Theme the redirection setting on the webform node form.
 */
function theme_webform_advanced_redirection_form($variables) {
  $form = $variables['form'];
  // Add special class for setting the active radio button.
  //$form['redirect_url']['#attributes']['class'] = array('webform-set-active');
  $form['redirect_url']['#attributes']['class'][] = 'webform-set-active';