Totally agree :+1: – however, unlike property mappings, I couldn't come up with a likely scenario in which we'd find a "new" property key in a subclass.

Since this method can't be cached, I'd like to have the default implementation be as cheap as possible. Using a black list allows us to default to `return nil;` quite easily.

I also didn't expect a deep hierarchy of `MTLJSONAdapter` subclasses but I guess you never know.

How about 

```objc
- (NSSet *)filterPropertyKeys:(NSSet *)propertyKeys forModel:(id<MTLJSONSerializing>)model {
    return propertyKeys;
}
```
?