Credentials unit tests

[tomjshore]

Types of changes

• 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)
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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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