set header for Authorization when doing QOAuth2AuthorizationCodeFlow::requestAccessToken()

Trying to play QtNetworkAuth with quizlet.

https://quizlet.com/api/2.0/docs/authorization-code-flow

Step 2: Request a token from our authorization server
An example HTTP request would look like:
POST /oauth/token HTTP/1.1
Host: api.quizlet.com
Authorization: Basic xxxx
Content-Type: application/x-www-form-urlencoded; charset=UTF-8
grant_type=authorization_code&code=GENERATED_CODE

The "Authorization" custom header is needed in this step, I don't know how should I add it into current QOAuth2AuthorizationCodeFlow::requestAccessToken().

Current workaround is something like:

request.setRawHeader(QByteArray("Authorization"), QByteArray("Basic xxxx"));

after

    query = QAbstractOAuthPrivate::createQuery(parameters);
    request.setHeader(QNetworkRequest::ContentTypeHeader,
                      QStringLiteral("application/x-www-form-urlencoded"));