Closure inside SwiftyStoreKit.completeTransactions(atomically: true) { ... } never gets reached

### Platform
iOS 13.1 and 13.2

### In app purchase type
- [ ] Consumable

### Environment
- [ ] Sandbox

### Version
SwiftyStoreKit (0.15.0) // installation: pod 'SwiftyStoreKit'

### Related issues

https://github.com/bizz84/SwiftyStoreKit/issues/432

https://stackoverflow.com/a/53162058/4833705

https://stackoverflow.com/q/53841379/4833705

### Issue summary
the `closure` inside `SwiftyStoreKit.completeTransactions(atomically: true) { ... }` never gets reached

### What did you expect to happen
I expected the closure to get entered

### What happened instead
the closure was ever entered

## Report

My app uses a tip jar for IAP so I only use Consumable.

I submitted my app to the App Store this morning and got a rejection saying they couldn't make an IAP when trying to make a Consumable purchase (a tip).

While testing as a sandbox tester when I checked the purchase I kept getting an error: "Unknown error. Please contact support":

```
SwiftyStoreKit.purchaseProduct(product, quantity: 1, atomically: true) { result in
switch result {
case .success(let product):
// fetch content from your server, then:
if product.needsFinishTransaction {

SwiftyStoreKit.finishTransaction(product.transaction)
}
print("Purchase Success: \(product.productId)")

case .error(let error):
switch error.code {
case .unknown:
print("Unknown error. Please contact support")
// failed cases ...
}
}
```

Upon further inspection even though I had this code below in `AppDelegate` the closure never gets called.

```
func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool {

// Doesn't enter closure
SwiftyStoreKit.completeTransactions(atomically: true) {

// *** it never reaches this point ***

(purchases) in

for purchase in purchases {
switch purchase.transaction.transactionState {
case .purchased, .restored:
if purchase.needsFinishTransaction {
// Deliver content from server, then:

let downloads = purchase.transaction.downloads
SKPaymentQueue.default().start(downloads)

SwiftyStoreKit.finishTransaction(purchase.transaction)
}
// Unlock content
case .failed, .purchasing, .deferred:
break // do nothing
}
}
}
return true
}
```

My sandbox tester email is confirmed, I signed out of the device as my real self, I logged in on the device as the sandbox tester, I'm logged into iCloud as the sandbox tester.

**What could be the reason for the closure not being entered in AppDelegate?**

Here's the code from the vc that I use to make the Consumable purchase:

```
var dataSource = [Tip]()
var sharedSecret = appStoreConnectSecretKey

let inAppProductIds = ["com.myCo.myAppName.firstTip", // 0.99
"com.myCo.myAppName.secondTip", // 9.99 ]

override func viewDidLoad() {
super.viewDidLoad()

getInAppPurchaseAmounts()
}

func getInAppPurchaseAmounts() {

// show spinner

let dispatchGroup = DispatchGroup()

for productId in inAppProductIds {

dispatchGroup.enter()

SwiftyStoreKit.retrieveProductsInfo([productId]) { [weak self](result) in
if let product = result.retrievedProducts.first {
let priceString = product.localizedPrice!
print("Product: \(product.localizedDescription), price: \(priceString)")

let tip = Tip(displayName: product.description,
description: product.localizedDescription,
productId: productId
price: priceString)

self?.addTipToDataSource(tip)

if let sharedSecret = self?.sharedSecret {

self?.verifyPurchase(with: productId, sharedSecret: sharedSecret)
}
dispatchGroup.leave()

} else if let invalidProductId = result.invalidProductIDs.first {
print("Invalid product identifier: \(invalidProductId)")
dispatchGroup.leave()

} else {
print("Error: \(String(describing: result.error))")
dispatchGroup.leave()
}
}
}

dispatchGroup.notify(queue: .global(qos: .background)) { [weak self] in
DispatchQueue.main.async { [weak self] in

// removeSpinnerAndReloadData()
}
}
}

func verifyPurchase(with productId: String, sharedSecret: String) {

let appleValidator = AppleReceiptValidator(service: .production, sharedSecret: sharedSecret)
SwiftyStoreKit.verifyReceipt(using: appleValidator) { result in
switch result {
case .success(let receipt):
let productId = productId
// Verify the purchase of Consumable or NonConsumable
let purchaseResult = SwiftyStoreKit.verifyPurchase(
productId: productId,
inReceipt: receipt)

switch purchaseResult {
case .purchased(let receiptItem):
print("\(productId) is purchased: \(receiptItem)")
case .notPurchased:
print("The user has never purchased \(productId)")
}
case .error(let error):
print("Receipt verification failed: \(error)")
}
}
}

func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
guard let cell = collectionView.cellForItem(at: indexPath) as? TipCell else { return }
guard let indexPath = collectionView.indexPath(for: cell) else { return }

let tip = dataSource[indexPath.item]

purchaseProduct(with: tip.productId)
}

func purchaseProduct(with productId: String) {

SwiftyStoreKit.retrieveProductsInfo([productId]) { result in
if let product = result.retrievedProducts.first {
SwiftyStoreKit.purchaseProduct(product, quantity: 1, atomically: true) { result in

switch result {
case .success(let product):
// fetch content from your server, then:
if product.needsFinishTransaction {
SwiftyStoreKit.finishTransaction(product.transaction)
}
print("Purchase Success: \(product.productId)")
case .error(let error):
switch error.code {
case .unknown:
print("Unknown error. Please contact support")
case .clientInvalid:
print("Not allowed to make the payment")
case .paymentCancelled:
print("Payment cancelled")
case .paymentInvalid:
print("The purchase identifier was invalid")
case .paymentNotAllowed:
print("The device is not allowed to make the payment")
case .storeProductNotAvailable:
print("The product is not available in the current storefront")
case .cloudServicePermissionDenied:
print("Access to cloud service information is not allowed")
case .cloudServiceNetworkConnectionFailed:
print("Could not connect to the network")
case .cloudServiceRevoked:
print("User has revoked permission to use this cloud service")
default:
print((error as NSError).localizedDescription)
}
}
}
}
}
}
```