A covenant by the mortgagor in a mortgage, “That no owner of the mortgaged property shall be entitled to any credit by reason of the payment of any tax thereon” shall be construed to mean that the mortgagor, for himself, his heirs, executors, administrators, successors and assigns, covenants with the holder of the mortgage that he and they will not, by reason of the payment of any tax, assessment or other municipal charge or imposition, claim any credit on account of the principal or interest due or to grow due on said mortgage or the bond or obligation intended to be secured thereby, and that should said mortgagor, his heirs, executors, administrators, successors or assigns, fail to pay any tax, assessment or other governmental charge, then and in that event the holder of the mortgage may at his option pay such tax, assessment or other governmental charge and the amount when so paid shall be added to and become part of the principal sum then due upon the bond or obligation referred to in said mortgage, and be secured by said mortgage and shall be payable on demand with legal interest.