Since this is primarily used for something like #264, I'd like to make this method private and instead offer

```objc
// A list of property keys that should not be serialized by the receiver.
//
// The default implementation returns nil, indicating that all property keys
// should be serialized.
//
// model - The model that is currently being serialized.
//
// Returns a set of properties that should not be serialized if this model is
// used in -JSONDictionaryFromModel:error:. May return nil.
- (NSSet *)ignoredPropertyKeysForModel:(id<MTLJSONSerializing>)model;
```

The output of `-JSONKeyPathsByPropertyKeyForModelClass:` could then be cached by `modelClass`.