No payment shall be made to creditors of any one class until all those of the preceding class have been fully paid, and if the assets are not sufficient to pay all the creditors of any one class, the creditors of such class shall be paid ratably; but a personal representative who, after 12 months from his qualification, pays a debt or demand of his decedent is not personally liable for any debt or demand against the decedent of an equal or superior class, whether it is of record or not, unless he had notice of such debt or demand before making such payment.