Consider moving ignoreNullValueOverwrites to the map, away from the mapper

[brockboland]

ignoreNullValueOverwrites is a property on VOKManagedObjectMapper, but most of the times I need to use it, it's because there's a particular property that isn't always included in the server response, so I don't want it to be overridden with a nil value…but other properties should be. If we moved that property to VOKManagedObjectMap—or added an equivalent there, to work alongside the existing property—a more fine-grained control could be achieved.