Add more tests and prop-types

[sindresorhus]

I didn't feel it was worth all the boilerplate and setup just for 4 super simple exports that I've manually tested well, but tests should be added when more things are added.

Should use AVA.

This task also includes adding prop types and default prop values to the types.

Inspiration: https://github.com/ReactTraining/react-router/tree/master/packages/react-router-dom/modules/__tests__

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[kishan3]

Hi, @sindresorhus Interested to contribute. Please let me know if any contribution guide is available.

[sindresorhus]

We have some tests now, but we could always use more! :)