Sets the due date of the invoice.
        @param value:date

def __set_due_date(self, value):
        '''
        Sets the due date of the invoice.
        @param value:date
        '''
        if type(value) != date:
            raise ValueError('Due date must be an instance of date.')

        if self.__date.date() and value < self.__date.date():
            raise ValueError("Due date cannot be anterior to the invoice " \
                             "date.")

        self.__due_date = value