Where any person is charged, before a court of summary jurisdiction, with an offence against the law of the Commonwealth, any exception, exemption, proviso, excuse, or qualification, whether it does or does not accompany the description of the offence in the section of the law creating the offence, may be proved by the person charged, but need not be specified or negatived in the information, and, if so specified or negatived, no proof in relation to the matter so specified or negatived shall be required on the part of the informant.