Ordered relationship deserialization to NSOrderedSet vs NSMutableArray

Since Core Data ordered models are modeled as NSOrderedSet,

it seems reasonable for reverseBlock to expect an NSOrderedSet* in the transformer function
```
+ (instancetype)reversibleTransformerWithForwardBlock:(MTLValueTransformerBlock)forwardBlock
                                         reverseBlock:(MTLValueTransformerBlock)reverseBlock
```
It's currently returning an NSMutableArray - is there a reason for this?