Fix google play purchases missing purchase date (#2703)

### Description 
Reported in RevenueCat/purchases-flutter#738. 

After #2654, we were
using the new Google Play identifiers that include the plan id in
expiration dates, but we didn't change the system that parses purchase
dates. Later on, when we map purchases, we iterate over the expiration
dates map, and since that id wasn't found in the purchase dates, it was
null, causing the issue in flutter.

This makes the purchase date calculation use the same algorithm that we
use for expiration dates to get the new google play product identifiers.

Sources/Identity/CustomerInfo+ActiveDates.swift

@@ -53,24 +53,24 @@ extension CustomerInfo {
                         .subscriptions
                         .lazy
                         .map { productID, subscription in
-                            let key: String
+                            let key = Self.productID(productID: productID, purchase: subscription)
                             let value = subscription.expiresDate
-
-                            // Products purchased from Google Play will have a product plan identifier (base plan)
-                            // These products get mapped as "productId:productPlanIdentifier" in the Android SDK
-                            // so the same mapping needs to be handled here for cross platform purchases
-                            if let productPlanIdentfier = subscription.productPlanIdentifier {
-                                key = "\(productID):\(productPlanIdentfier)"
-                            } else {
-                                key = productID
-                            }
                             return (key, value)
                         }
                 )
     }
 
     static func extractPurchaseDates(_ subscriber: CustomerInfoResponse.Subscriber) -> [String: Date?] {
-        return subscriber.allTransactionsByProductId.mapValues { $0.purchaseDate }
+        return Dictionary(
+                    uniqueKeysWithValues: subscriber
+                        .allPurchasesByProductId
+                        .lazy
+                        .map { productID, purchase in
+                            let key = Self.productID(productID: productID, purchase: purchase)
+                            let value = purchase.purchaseDate
+                            return (key, value)
+                        }
+                )
     }
 
 }
@@ -79,6 +79,17 @@ extension CustomerInfo {
 
 private extension CustomerInfo {
 
+    static func productID(productID: String, purchase: CustomerInfoResponse.Subscription) -> String {
+        // Products purchased from Google Play will have a product plan identifier (base plan)
+        // These products get mapped as "productId:productPlanIdentifier" in the Android SDK
+        // so the same mapping needs to be handled here for cross platform purchases
+        if let productPlanIdentfier = purchase.productPlanIdentifier {
+            return "\(productID):\(productPlanIdentfier)"
+        } else {
+            return productID
+        }
+    }
+
     static func referenceDate(for requestDate: Date) -> (Date, inGracePeriod: Bool) {
         if Date().timeIntervalSince(requestDate) <= Self.requestDateGracePeriod.seconds {
             return (requestDate, true)

Sources/Networking/Responses/CustomerInfoResponse.swift

@@ -233,6 +233,7 @@ extension CustomerInfoResponse.Subscriber {
         return self.allPurchasesByProductId.mapValues { $0.asTransaction }
     }
 
+    // This returns objects of type `Subscription` but also includes non-subscriptions
     var allPurchasesByProductId: [String: CustomerInfoResponse.Subscription] {
         let subscriptions = self.subscriptions
         let latestNonSubscriptionTransactionsByProductId = self.nonSubscriptions

Tests/UnitTests/Purchasing/CustomerInfoTests.swift

@@ -58,7 +58,8 @@ class BasicCustomerInfoTests: TestCase {
                     "expires_date": "2100-07-30T02:40:36Z",
                     "period_type": "normal",
                     "is_sandbox": false,
-                    "product_plan_identifier": "monthly"
+                    "product_plan_identifier": "monthly",
+                    "purchase_date": "2018-05-20T06:24:50Z"
                 ],
                 "onemonth": [
                     "expires_date": BasicCustomerInfoTests.expiredSubscriptionDate,
@@ -447,6 +448,11 @@ class BasicCustomerInfoTests: TestCase {
         expect(purchaseDate) == Date(timeIntervalSince1970: 1526797490)
     }
 
+    func testPurchaseDateForGooglePlayProductIdentifier() throws {
+        let purchaseDate = try XCTUnwrap(self.customerInfo.purchaseDate(forProductIdentifier: "gold:monthly"))
+        expect(purchaseDate) == Date(timeIntervalSince1970: 1526797490)
+    }
+
     func testPurchaseDateEmpty() throws {
         let response = [
             "request_date": "2019-08-16T10:30:42Z",