Make Object.prototype.toSting() unenumerable

While I strongly agree that `toSting()` should be part of all major
browser's spec, I don't think it should be enumerable.

For example:

```javascript
/* Run Object.toSting shim here. */
var test = [1];

for (var index in test) {
   console.log(index);
}

// will log: "0", "toSting"
```

Author: TheIronDev