Returns a DOM representation of the deliveries method
        @returns: Element

def to_xml(self):
        '''
        Returns a DOM representation of the deliveries method
        @returns: Element
        '''
        for n, v in { "method": self.method, "status": self.status,
                     "date":self.date}.items():
            if is_empty_or_none(v):
                raise DeliveryMethodError("'%s' attribute cannot be " \
                                       "empty or None." % n)

        doc = Document()
        root = doc.createElement("delivery")
        super(DeliveryMethod, self).to_xml(root)
        self._create_text_node(root, "method", self.method)
        self._create_text_node(root, "status", self.status)
        self._create_text_node(root, "reference", self.ref, True)
        self._create_text_node(root, "date", self.date)
        return root