[Sys.Xml] Add unit test for Novell bug #594490

The unit test demonstrates the problem of serializing the
xml:lang attribute.
[Sys.Xml] Allow qualified names in XmlAttribute serialization

This fixes Mono bug #594490
(https://bugzilla.novell.com/show_bug.cgi?id=594490).
Merge pull request #921 from ermshiperete/novell-bug-594490

Allow qualified names in XmlAttribute serialization