fix(keys): skip key transformation for non-plain objects like Date (#356)

[MohammadYusif]

Summary

• changeKeys treated Date (and any non-plain object) as a plain object to recurse into, because the isObject guard only checked !== null && typeof === "object"
• This caused Date values to become empty {} objects, breaking prototype methods like .toISOString()
• Non-plain objects (Date, RegExp, Map, Set, class instances) should be returned as-is

Changes

• packages/change-case/src/keys.ts: added isPlainObject helper checking the prototype chain; added if (!isPlainObject(object)) return object guard after the array branch
• packages/change-case/src/keys.spec.ts: added two regression tests for Date property and mixed Date + string property objects

Fixes #356