…mpute_engine._metadata.get() (#398)

Initial fix of issue #211 was done in CL #323, but only for .ping()
This one is adding same behaviour & tests for .get() method, as the problem still occurres
See the issue for details

Refs: #323
Resolves: #211

#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.