Added support for native XML nodes when serialising

Hey as per our discussion I could not find a way around the use of the if. The good news is it should hopefully not cause a problem because now it says if you want to pass a native Nokogiri node then we trust what you've placed in there and will not manually serialise for you. Thoughts?
Yes! :grin: I think the CDATA option is a valid extension to the existing API. In addition to that, we could make the Nokogiri-return value generic! I'll have a look at it after work.