Updates the expiration time of a token.

def set_exp(self, claim='exp', from_time=None, lifetime=None):
        """
        Updates the expiration time of a token.
        """
        if from_time is None:
            from_time = self.current_time

        if lifetime is None:
            lifetime = self.lifetime

        self.payload[claim] = datetime_to_epoch(from_time + lifetime)