Random errors when switching between 2 types of subscriptions

### Platform

- [ ] iOS

### In app purchase type

- [ ] Auto-Renewable Subscription

### Environment

- [ ] Sandbox

### Version
0.8.4 (maybe 0.8.5 too)

## Report

### Issue summary
Implemented as in your example from readme:
1)buy subscription1 and wait while it is applied
2)buy subscription2

### What did you expect to happen
Success result or something like "changed subscription"

### What happened instead
Step 1 - successful subscription. Step 2 - random errors returned by library - "Cancelled" or "Unknown"