Empty prototype is never added to the property list thanks to check at L157.

These two lines are filtering out `__proto__` properties that were added manually by the developer, which is misleading and confusing. Please remove these two lines and add a test verifying `__proto__` properties are preserved.

Here is a sample code:
```js
var obj = Object.create(null);
obj.__proto__ = null;
```

Please read the full discussion in joyent/node#6630 to understand why the code above creates a "regular" property in node v0.11 instead of modifying the prototype link.