Pretty often purchaseProduct returns: Unknown error. Please contact support

### Platform

- [x ] iOS
- [ ] macOS
- [ ] tvOS

### In app purchase type

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

### Environment

- [ x] Sandbox
- [ ] Production

### Version
ℹ Please replace this with the version of SwiftyStoreKit you're using.
0.8.4

### Related issues
ℹ Please replace this with references to similar issues (either open or already closed).

## Report

### Issue summary
I migrated from 0.6.1 which worked fine and now I'm pretty often experiencing this error while testing. Is this a know issue?

### What did you expect to happen
Purchase should succeed

### What happened instead
Unknown error occurs.