Purchase tracking fix variable type to be string

* SKProduct.price is an NSDecimalNumber and we had been sending this as is, but the backend expects a String
* This can be reproduced manually by a creating purchase array and calling `OneSignalUserManagerImpl sendPurchases`
* It returns a 400 with: "Field deltas.purchases.amount was expecting value of type 'string', received value of type 'number' instead"

Author: nan-li

iOS_SDK/OneSignalSDK/Source/OneSignalTrackIAP.m

@@ -124,8 +124,14 @@ - (void)productsRequest:(id)request didReceiveResponse:(id)response {
         NSString* productSku = [skProduct performSelector:@selector(productIdentifier)];
         NSMutableDictionary* purchase = skusToTrack[productSku];
         if (purchase) { // In rare cases this can be nil when there wasn't a connection to Apple when opening the app but there was when buying an IAP item.
+            
+            // SKProduct.price is an NSDecimalNumber, but the backend expects a String
+            NSNumberFormatter *formatter = [NSNumberFormatter new];
+            [formatter setMinimumFractionDigits:2];
+            NSString *formattedPrice = [formatter stringFromNumber:[skProduct performSelector:@selector(price)]];
+
             purchase[@"sku"] = productSku;
-            purchase[@"amount"] = [skProduct performSelector:@selector(price)];
+            purchase[@"amount"] = formattedPrice;
             purchase[@"iso"] = [[skProduct performSelector:@selector(priceLocale)] objectForKey:NSLocaleCurrencyCode];
             if ([purchase[@"count"] intValue] == 1)
                 [purchase removeObjectForKey:@"count"];