Object.setPrototypeOf sham

As discussed already in [this thread](https://github.com/paulmillr/es6-shim/pull/223#issuecomment-51696022) here a cross browser/platform `Object.setPrototypeOf` that works reliably **requiring** reassignment of the same object, meaning external, non own objects, might not be upgraded if the engine was not compatible.

As sham, this does not require `es6-shim` as is.
What if setPrototypeOf is implemented natively? Should this be checking for an existing implementation?