Should optionally support NSNull for nil valued keys

[hermiteer]

This would allow for testing dictionary vs declare properties to correctly test if some values are NSNull rather than entirely missing.

This would require updating the test methods, as well as setObject:forSetter: and any returning object methods.

[hermiteer]

Actually a better way to do this would be to purge those values when instancing with a dictionary. A static method can be provided for subclasses to use, or a static flag to enable it for all instances of CottonObject, or a method to overload to enable it on a per class basis.