Implementing write chapter

i think the equivalent in doctrine-dbal would be [propsToXML](https://github.com/jackalope/jackalope-doctrine-dbal/blob/master/src/Jackalope/Transport/DoctrineDBAL/Client.php#L917) which looks a lot more complicated. i am a bit afraid of you relying on converting the type "to itself" actually doing anything - is that not a noop?

what format are you storing the data in? what if we would move the to xml method to the base transport and you use xml here?