also:
* updated rubocop and applied styling
* made small test refactorings (using the expect block notation)
* made a small refactor to remove an unnecessary ``return credentials``
* bumped the version
- if the ServiceAccountCredentials#apply is called when the scope is not set,
authentication with the equivalent ServiceAccountJwtHeaderCredentials instance
is attempted.
- adds no new functionality beyond the initial attempt at providing
Usable Auth constructors.
- adds enough packaging metadata to be able to export this library as a gem.
- next steps:
complete usable auth implemntation of Application Default Credentials.