No CData

I've defaulted the cdata to true to keep BC with existing serializer metadata. Although it's a bit annoying setting it to false on most of the string properties.

Here is one another thing I'd like to implement ability for Handler to override/specify the cdata property. 
There are two use cases:
* when serializing a datetime
* when serializing custom object for which the handler knows that there is no need to be enclosed inside cdata

Do we add a new method only into the XMLSerializationVisitor class?