When any action or prosecution shall be brought or any proceedings held 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.
     Such plea shall be proved by the production of the warrant directing the act, and purporting to be signed by such Magistrate and the defendant shall thereupon be entitled to a decree in his favour, notwithstanding any defect of jurisdiction in such Magistrate. No proof of the signature of such Magistrate shall be necessary, unless the Court shall see reason to doubt its being genuine:
     Proviso.—Provided always that any remedy which the party may have against the authority issuing such warrant shall not be affected by anything contained in this section.