Section 15. Cities, towns and districts authorized by special acts to incur debt shall not be exempt from liability to pay debts contracted for the purposes for which they may lawfully expend money under such acts, notwithstanding that the amount of the annual payment and the period of the loan may not have been specified by the vote authorizing the debt to be incurred, and the officers authorized to issue bonds or notes therefor may issue and same, subject to the provisions and limitations of such acts, provided that in all other respects the requirements of law shall have been complied with; and the provisions of this section shall apply to any bonds or notes of a city, town or district issued under the authority of any special act heretofore or hereafter enacted.