if typed, set the xsi:type attribute 
        Paramters:
            el -- MessageInterface representing the element

def set_attribute_xsi_type(self, el, **kw):
        '''if typed, set the xsi:type attribute 
        Paramters:
            el -- MessageInterface representing the element
        '''
        if kw.get('typed', self.typed):
            namespaceURI,typeName = kw.get('type', _get_xsitype(self))
            if namespaceURI and typeName:
                self.logger.debug("attribute: (%s, %s)", namespaceURI, typeName)
                el.setAttributeType(namespaceURI, typeName)