Making the current environ a tweakable setting.
Moving GoogleCredentials.get_application_default into helper.
Refactoring _get_implicit_credentials into component pieces.
Using require_refresh in _get_credential_from_file.

Also renaming application_default_credential as credential.
Adding simple test for user-auth.

Shows code as-is indicates refresh fails. Need to initiate
flow.
Showing discrepancy between client_secrets and already finished.
Adding hack that actually works for user-implicit auth.
Removing unused part of test.