refix issue 5 in es5.sham.js

Hi,

i use your shim in my libary in need the fallback implementation in ie8 for the getOwnPropertyDescriptor method. With this fix the sham works also in ie8 on none DOM objects as it was fixed in the issue #5 pull rquest in the past.

I hope that you will reintegrate the fix again in your new code base ;-)