Per-request AddCredential not working with BatchRequest

With the per-request credential example provided by @amanda-tarafa in #2064, I was able to get it working for services like Calendar and Oauth2. But I'm having trouble with `GmailService`.

I use `GmailService` via `BatchRequest`, like this

```
var cred = GoogleCredential.FromAccessToken(myToken);

var batchRequest = new BatchRequest(_gmailService);

foreach (var gmailMessage in gmailMessages)
{
batchRequest.Queue
(_gmailService.Users.Messages.Send(gmailMessage, "me").AddCredential(cred), myCallback));
}

await batchRequest.ExecuteAsync();
```
I'm getting error:

> Error 401, Request is missing required authentication credential. Expected OAuth 2 access token, login cookie or other valid authentication credential. See https://developers.google.com/identity/sign-in/web/devconsole-project.