bug: serialization with defaultNS fails

[Jan Kowalleck]

serializer does not add namespaces to XML-attributes, so setting a defaultNS did not work.

the following will fail:

ElementTree.tostring(
  foo.as_xml(view_=_view, as_string=False, xmlns='myNS')
  method='xml',
  encoding='unicode', xml_declaration=True,
  default_namespace='myNS'
)