fix: default value setting to correctly resolve function return types

[yutak23]

This ought to fix #116.

The problem lies in the generic type OptionsType.
For example, if pascalCase is not included in the options, the following type definition would result in the type being unknow.

OptionsType['pascalCase']; // unknown

Therefore, the type would be CamelCaseKeys<T, true, unknown, unknown, unknown, ""> as in the issue, resulting in a compile error.

With this fix, i change the type to be undefined when there is no key in the options, so the type WithDefault works correctly and the type of the default value part is the default value for each.

Specifically, it becomes CamelCaseKeys<T, true> when the options are as follows.

camelcaseKeys(response, {deep: true}); // CamelCaseKeys<T, true>

※The above without abbreviation would be CamelCaseKeys<T, true, false, false, [], [], ''>.

[sindresorhus]

Thanks for the quick fix 🙏

[WoodyWoodsta]

Thank you @yutak23!