Stricter options

[codyzu]

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

This PR makes aligns the implementation with the documentation of the origin option:

• any value of the origin option or return value of an origin function that is falsy but not false, results in an error
• a non false falsy value is not defined in the documentation, and will now be considered an error
• this aligns the behavior if origin is a value or a function
• before a falsy value of the origin option disabled CORS, but a falsy non false return value of an origin function set the allowed origin to * (any origin)

This is a follow-up PR to #99 . This PR allowed #99 to only refactor and not change functionality.

[codyzu]

@mcollina this should be the last PR to improve the maintainability of this module

[zekth]

lgtm