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?
So I understand you correctly. We want a mechanism to wrap any raw content in a CDATA tag when the cdata option is passed in as true? So if cdata is true then we have a separate binder altogether? But do we still want to support native Nokogiri types much like in the test example?