Fixed concurrency issue on iOS. (#413)

The `productsRequest` method as per Apple docs can be called on any
thread (source:
https://developer.apple.com/documentation/storekit/skproductsrequestdelegate
(please check text in read)).
Working with `validProducts` on such thread can cause concurrency
issues, such as this one:
**"Terminating app due to uncaught exception 'NSGenericException',
reason: '*** Collection <__NSArrayM: 0x283ca4090> was mutated while
being enumerated.'"**

Moreover, flutter results must be called on platform main thread only
(source:
https://docs.flutter.dev/development/platform-integration/platform-channels)
_"Even though Flutter sends messages to and from Dart asynchronously,
whenever you invoke a channel method, you must invoke that method on the
platform’s main thread."_

To fix these issues I simply wrapped all the code in
`dispatch_async(dispatch_get_main_queue, ^{ ... })`.
Please pull this fix.

Co-authored-by: Alfred Lapkovsky <[email protected]>

ios/Classes/FlutterInappPurchasePlugin.m

@@ -286,21 +286,23 @@ - (void)request:(SKRequest *)request didFailWithError:(NSError *)error {
 }
 
 - (void)productsRequest:(nonnull SKProductsRequest *)request didReceiveResponse:(nonnull SKProductsResponse *)response {
-    NSValue* key = [NSValue valueWithNonretainedObject:request];
-    FlutterResult result = [fetchProducts objectForKey:key];
-    if (result == nil) return;
-    [fetchProducts removeObjectForKey:key];
+    dispatch_async(dispatch_get_main_queue(), ^{
+        NSValue* key = [NSValue valueWithNonretainedObject:request];
+        FlutterResult result = [self.fetchProducts objectForKey:key];
+        if (result == nil) return;
+        [self.fetchProducts removeObjectForKey:key];
+
+        for (SKProduct* prod in response.products) {
+            [self addProduct:prod];
+        }
+        NSMutableArray* items = [NSMutableArray array];
 
-    for (SKProduct* prod in response.products) {
-        [self addProduct:prod];
-    }
-    NSMutableArray* items = [NSMutableArray array];
-
-    for (SKProduct* product in validProducts) {
-        [items addObject:[self getProductObject:product]];
-    }
+        for (SKProduct* product in self->validProducts) {
+            [items addObject:[self getProductObject:product]];
+        }
 
-    result(items);
+        result(items);
+    });
 }
 
 -(void)addProduct:(SKProduct *)aProd {