fix(keys): preserve non-plain object values like Date and RegExp

[naveentehrpariya]

Fixes #356.

Problem

change-case/keys recurses into any value where typeof value === "object". That includes non-plain objects such as Date, RegExp, Map/Set and class instances. For those, it creates a new object with the same prototype and copies the own enumerable keys — but their state lives in internal slots, not enumerable keys, so the result is a broken object.

import { camelCase } from "change-case/keys";

const out = camelCase({ created_at: new Date() }, 2);
out.created_at.toISOString();
// TypeError: Method Date.prototype.toISOString called on incompatible receiver [object Object]

Fix

Only recurse into plain objects (prototype Object.prototype or null) and arrays. Any other object is returned untouched, so Date, RegExp, etc. pass through as-is.

Tests

Added cases asserting that a Date/RegExp value is preserved by reference (and remains usable), and that a non-plain object passed as the top-level value is returned untouched. Both fail without this change and pass with it; the full package suite (29 tests) stays green.