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?
We also need to support parsing from CDATA, so maybe we could introduce something as

```ruby
property :title, cdata:true
```

That would also remove the `if` in `serialize_node` as we could use a separate Binding?