Refunding bonds shall be special obligations of the issuing municipality and shall be payable from, and secured by a lien upon, the revenues of the enterprise as shall be described more fully in the resolution of the governing body authorizing the issuance of the refunding bonds. Having due regard to the cost of operation and maintenance of the enterprise and the amount or proportion, if any, of the revenues of the enterprise previously pledged, a municipality, by a resolution of its governing body, may pledge for the security of the refunding bonds a fixed amount without regard to any fixed proportion of the gross revenues of the enterprise.