Updated before_create :generate_confirmation_token callback

:generate_confirmation_token method not only creates a token, but also sets the resource.confirmation_sent_at attribute. When :skip_confirmation_notification is specified, we don't want the resource.confirmation_sent_at attribute to be filled. Thus, a condition is added to the before_create :generate_confirmation_token! callback such that it's only invoked if :send_confirmation_notification? is true