fixed issue #242 and update an test case

- 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.
revert test case changes
fixed additional failing cases
introduced to Object.keys on FF3.6 & Opera
change string enumerable check to owns('x', '0')
Merge pull request #285 from eriklynd/master

Fix `Object.keys` issue with boxed primitives with extra properties in older browsers.

Fixes #242.