When any action, prosecution, or proceeding shall be brought against any Police-officer for any act done by him in such capacity, it shall be lawful for him to plead that such act was done by him under the authority of a warrant issued by a Magistrate; and such plea shall be proved by the production of the warrant directing the act and purporting to be signed by a Magistrate. And the defendant shall, thereupon, be entitled to a decree in his favour, notwithstanding any defect of jurisdiction in such Magistrate. And no proof of the signature of such official shall be necessary, unless the Court shall see reason to doubt it being genuine:     Proviso.- Provided always that any remedy which the party may have against the authority issuing such warrant shall remain entire.