fixed issue #242

- issue #242: Object.keys shim does not work with boxed primitives with extra properties.
- update test case in `s-object.js': enumerating over non-enumerable properties.

And I have a question about the test cases I have modified. 

I am not very clear why the `prototype` s are used instead of the instance object. Apparently on legacy browsers which `Object.defineProperty` is not supported, shim functions are directly added to the prototypes. And it will cause the test cases to fail. Would it be better to test against the blank objects? 

Thanks.