avoiding throw if the key passed into WeakMap->get is an object created using `Object.create(null)`

we are trying to use the `harmony` polyfill to get https://github.com/jorendorff/js-loaders to work in all major browsers, and the `harmony` polyfill really helps, but we found a very small issue affecting the WeakMap implementation when passing objects without `prototype` as the `key` when calling `get`. This patch seems to work fine for us.