ModelFilters and closures should be able to replace an object with a list

[alexvigdor]

When evaluating closures for deferred resolution of model branches in XML and JSON writers, and when using ModelFilter.transform() or ModelFilter.invoke() to substitute values in a model, it should be possible to replace an object value with a list or null value. However this currently does not work as all these cases only support passing a non-null Object on to their consumer; we should add support and tests for the list and null use cases.