Update RKManagedObjectStore.m

[YevgenStarikov]

Fixing the following NEW error: "this class is not key value coding-compliant for the key @count" for iOS 10+

[YevgenStarikov]

@razmara this PR to align with source code and fix error

[YevgenStarikov]

@razmara FYI current code looks:

NSMutableSet <NSManagedObjectID *> *objectIDs = [NSMutableSet set];

    void (^unionObjectIDs)(NSMutableSet *, NSSet *) = ^(NSMutableSet *objectIDs, NSSet *objects) {
        if (objects != nil) {
            [objectIDs unionSet:[objects valueForKey:NSStringFromSelector(@selector(objectID))]];
        }
    };

    unionObjectIDs(objectIDs,notification.userInfo[NSInsertedObjectsKey]);
    unionObjectIDs(objectIDs,notification.userInfo[NSUpdatedObjectsKey]);
    unionObjectIDs(objectIDs,notification.userInfo[NSDeletedObjectsKey]);

but I think it is OK to proceed with this PR

[sasan-soroush]

Thanks @YevgenStarikov.