All claims existing against the estate at the beginning of the proceedings in insolvency may be proved and allowed as of that date, whether then payable or not, with a rebate of interest upon such claims as are not then payable and do not bear interest. Costs taxable against the debtor in an action then pending in which he is plaintiff and which the assignee declines to prosecute and taxable costs incurred in good faith by a creditor before the beginning of the proceedings in an action brought against the debtor to recover a provable debt may also be proved and allowed.