* Refactor env var methods
Have the iteration return for the first match, instead of comparing
all elements and choosing the first one that matches.
* Combine file and json credentials detection
Allow an environment variable to contain either a file path
or JSON to describe the credentials.
This change matches how these variables are used in Google Cloud.
If no PATH_ENV_VARS were found Credentials.default would return an empty array.
Update each default lookup method to return nil if no match was found.
Fix call to from_default_paths method, was calling from_default_vars instead.
Add spec coverage for path, json, and default cases.
Add spec coverage for application_default case.
Fix spec coverage to check the return type from calling Credentials.default.
Mike Moore
Graham Paye
Daniel Azuma
Thea Flowers
Ernest Landrito