Fix XML serialize and xml_eql in IE9

Use the xml property instead in IE for serializing XML.
The xml_eql tests were failing in IE9 on a difference between null and an
empty string for namespaceURI of attributes, this difference is not
relevant to our testing so make sure we ignore.
implement review comment by @ahocevar
Merge pull request #734 from bartvde/xml-ie9

Fix XML serialize and xml_eql in IE9 (r=@ahocevar)