Use http client in validateReceiptIos

[dev-yakuza]

The validateReceiptIos function uses the http library directly instead of the _client instance.

https://github.com/dooboolab/flutter_inapp_purchase/blob/24b290cff01d99494e7a38fb27373027acf8cd07/lib/flutter_inapp_purchase.dart#L552-L560

And the validateReceiptAndroid function uses the _client instance.

https://github.com/dooboolab/flutter_inapp_purchase/blob/24b290cff01d99494e7a38fb27373027acf8cd07/lib/flutter_inapp_purchase.dart#L588-L594

I think using the _client is reasonable because we can use it for testing by assigning the mock client.

final client = MockClient();

FlutterInappPurchase(FlutterInappPurchase.private(
  FakePlatform(operatingSystem: 'ios'),
  client: client,
));

So, I changed http to _client and initialize it.

[hyochan]

Superb detail 💯