Validity of taxes and sale presumed unless attacked in answer.

In an action to foreclose the right of redemption, the plaintiff shall not be required to plead or prove the various steps, procedure, and notices for the assessment and imposition of the taxes for which the real property was sold or the proceedings taken by the Mayor to sell the real property. The validity of the procedure is conclusively presumed unless a defendant in the proceeding shall, by answer, plead as an affirmative defense, the invalidity of the taxes, the invalidity of the proceedings to sell, or the invalidity of the sale.