-disconnectWithCompletion: sends the revocation request over GET, because startFetchURL: builds an NSMutableURLRequest without setting HTTPMethod. Our documentation ([mobile](https://developers.google.com/identity/protocols/oauth2/native-app#tokenrevoke)) says to use a POST request; and [RFC 7009 section 2.1](https://datatracker.ietf.org/doc/html/rfc7009#section-2.1) specifies a POST carrying the token in a form-encoded body.
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