Ruby: Add unit test coverage for credentials.rb

[geigerj]

What

The new credentials.rb file does not have autogenerated test coverage. We should add coverage, will would be able to catch issues like https://github.com/GoogleCloudPlatform/google-cloud-ruby/issues/1837. This could either involve modifying the existing autogenerated unit tests to mock at the googleauth layer rather than at the Credentials class layer, or it could involve generating a new unit test for this file.

cc: @landrito