Webform form settings cannot be saved due to text format bug

The Drupal 7 #text_format property is great but it's causing a problem with saving our Webform settings in Drupal 7. Right now we're using a text format on the confirmation message, but we get an error on save because $form_state['values']['confirmation'] is an array, not a string. This patch corrects this and allows saving in Drupal 7.