LibJS: Fix Object.prototype.valueOf() behavior

No idea why it was implemented like this, but a value_of() call on the
coerced object is not part of the spec. Also added some tests.

Userland/Libraries/LibJS/Tests/builtins/Object/Object.prototype.valueOf.js

@@ -0,0 +1,8 @@
+test("basic functionality", () => {
+    expect(Object.prototype.valueOf).toHaveLength(0);
+
+    const o = {};
+    expect(o.valueOf()).toBe(o);
+
+    expect(Object.prototype.valueOf.call(42)).toEqual(new Number(42));
+});