Whenever a city of the first class lawfully reannexes territory which it had formerly annexed but which annexation was illegal because the statutes under which such original annexation was made were unconstitutional and void, (1) all contracts for public improvements, warrants and bonds issued by the city of the first class with respect to such territory and all payments made thereon shall thereby be validated, binding and legal upon such city of the first class in the same manner and with the same effect as if the original annexation had been lawful, (2) all obligations of any sanitary and improvement district assumed by a city of the first class with respect to such territory shall thereby be validated, binding and legal upon such city of the first class in the same manner and with the same effect as if the original annexation had been lawful, and (3) such city of the first class may issue bonds under the appropriate statutes relating to public improvements to refund the warrants, warrant interest and any unpaid cost with respect to public improvements referred to in subdivision (1) of this section in the same manner and with the same effect as if the original annexation had been lawful.