Download of In App Purchase cancels/stops

### Platform

- [X] iOS
- [ ] macOS
- [ ] tvOS

### In app purchase type

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

### Environment

- [X] Sandbox
- [ ] Production

### Version
ℹ 0.13.3

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

## Report

### Issue summary
When I start a download via SwiftyStoreKit.start(downloads) and close the app so that it enters background mode (just pressing the home button once) the download won't continue when I open the app again. Even creating a global variable that contains all downloads objects and pausing downloads inn Appdelegate at "didenterbackground" and resuming the downloads when "didenterforeground" didnt work. How can I solve this or more specifically how can I have the download continue in background even if the app is closed?

### What did you expect to happen
ℹ Downloads resume or continue to run in background.

### What happened instead
ℹ Download didnt continue. It just stuck without an error.