Refactor SetNonNullable Type

[joealden]

Resolves #486.

[sindresorhus]

Would you be able to add a type test? So we can ensure it doesn't regress in the future.

[joealden]

@sindresorhus I've given this a try in e0f5d9d, but I'm not totally sure how to write a test that tests for this case, as we aren't wanting to test the resulting type, we are instead wanting to test that the type declaration itself (within a function declaration) doesn't throw an error. If the current codes doesn't suffice, could you please take a stab at this?

[sindresorhus]

Thanks :)