Updating a model with null children values breaks the bindings

[lisandropuzzolo]

Updating with a null value a model like this:

model = ko.mapping.fromJS({ person:{ name: 'john'
} });

updating it with {person: null} will break the bingings: http://jsfiddle.net/vJhj8/14/ ¿Is this the expected behaviour?

Here is another fiddle in wich you can see that when updating the model with a null value, the "create" callback is executed instead of "update" callback: http://jsfiddle.net/JKw6f/