Define default roll function to be called in apply method.

def month_roll(self):
        """
        Define default roll function to be called in apply method.
        """
        if self._prefix.endswith('S'):
            # MonthBegin
            roll_func = self.m_offset.rollback
        else:
            # MonthEnd
            roll_func = self.m_offset.rollforward
        return roll_func