All bonds shall be signed by the chairman of the authority, shall be attested by the secretary thereof, and shall bear the official seal of the authority. Any coupons attached thereto shall bear the signature of the chairman of the authority and, if the resolution authorizing the issuance of the bonds so provides, may be attested by the secretary of the authority. Any coupon may bear the facsimile signature of such persons. Any bond may be signed, sealed, and attested on behalf of the authority by such persons as at the actual time of the execution of such bonds shall be duly authorized to hold the proper office, although at the date of such bonds such persons may not have been so authorized or shall not have held such office. In case any officer whose signature appears on any bonds or whose facsimile signature appears on any coupon ceases to be such officer before the delivery of such bonds, such signature shall nevertheless be as valid and sufficient for all purposes as if he had remained in office until such delivery.