Always carry last_request_uri and last_request_at even if the last_*
        values are null.

def _last_values(self):
        """Always carry last_request_uri and last_request_at even if the last_*
        values are null.

        """
        if not self.session.get('last_request_uri'):
            self.session.last_request_uri = None
        self.session.last_request_at = self.session.get('last_request_at', 0)