xml:lang not handled

[gsnedders]

http://code.google.com/p/html5lib/issues/detail?id=154

Reported by @fantasai, Jun 1, 2010

What steps will reproduce the problem?
Try to reserialize a document containing xml:lang

What is the expected output? What do you see instead?
For HTML output, I'd expect to get lang= in place of xml:lang=.
Instead I get {http://www.w3.org/XML/1998/namespace}lang which
is completely malformed.

[gsnedders]

IMO this falls into the realms of rewriting the tree — out of scope for the serializer. At the very least, however, DataLossWarning should be triggered, instead of "just" returning garbage.