fix: always pass body of type bytes to `google.auth.transport.Request` (#421)

[`google.auth.transport.Request`](https://google-auth.readthedocs.io/en/latest/reference/google.auth.transport.html#google.auth.transport.Request) says that the body should be of type bytes. Some of our code was passing in strings as reported in #318.

In practice this was not causing issues, as the two http transports [requests](https://google-auth.readthedocs.io/en/latest/reference/google.auth.transport.requests.html) and [urllib3](https://google-auth.readthedocs.io/en/latest/reference/google.auth.transport.urllib3.html) are able to handle bodies passed as strings.
4 files changed