Returning an error when switching to another subscription plan within the same group

### Platform

- [v] iOS
- [ ] macOS
- [ ] tvOS

### In app purchase type

- [ ] Consumable
- [ ] Non-consumable
- [v] Auto-Renewable Subscription
- [ ] Non-Renewing Subscription

### Environment

- [v] Sandbox
- [v] Production

### Version
0.10.7

### Related issues

## Report

### Issue summary
When the subscription changes to another plan, it returns unknown-error.

Steps to reproduce
1. purchase an auto-renewable subscription
2. switch to another plan within the same group
3. it returns error even though the os says it succeeded

### What did you expect to happen
it returns success

### What happened instead
it returns an error

Recently, Apple added a feature the user can change their subscription plan within the same group.
But it seems like this library doesn't cover the case.
Please fix this!