Credentials unit tests

[tomjshore]

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [x] I have read the CONTRIBUTING document.

This is to add unit tests to the NetflixCredentials class. The only change to "real" code was to change the if statements on string validation so the when encoding/decodeing it will return empty string if email or password is empty (this seems to be the intention viewing the code comments).

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Changes to Core Features:

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