google.auth.transport.Request is called with type(body) == str #318

According to eg https://google-auth.readthedocs.io/en/latest/reference/google.auth.html#google.auth.default, classes implementing google.auth.transport.Request will be called with a body parameter of type bytes. This makes sense, because the request agent is in no position to decide how a str body should be encoded for transport.

Unfortunately, in some cases the googl.auth module does pass str value. One such call is made in