IPv6 Network Rejection & Network Error

### 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 0.13.1

### Related issues
https://github.com/bizz84/SwiftyStoreKit/issues/81 **IPv6 network issue**
https://github.com/bizz84/SwiftyStoreKit/issues/276 **Non-Consumable: IPv6 App Store Review Rejection.**

## Report

### Issue summary
ℹ I can't get any error with the sandbox user, every step of purchase is going well but when I send it to review to the app store, the error that they got caused the rejection. The app throws a network error when the app launched, that I don't face it in any step on my test device & sandbox user.

I read all the comments on other related issues, but I couldn't find any clue to solve this problem.
Thank you.

> Hello,
>
> Thank you for your response.
>
> Regarding the 2.1.0 guideline, we discovered one or more bugs in your app when reviewed on iPad and iPhone running iOS 11.3.1 on Wi-Fi connected to an IPv6 network.Specifically, when we launched the app, we received an error alert. We were unable to review the app content nor the In-App Purchase products.
>
> It would be appropriate to run your app on a device to identify any issues, then revise and resubmit your app for review.


### What did you expect to happen
ℹ I want to fix the unexpected error alert when in the review.

### What happened instead
ℹ No error when the app launched.