UglifyJS compressing causes Range Error

[UglifyJS2] with compression enabled and default options will see the line

```javascript
"" + this.toString();
```

and minify it into this line

```javascript
"" + this
```

and that normally should work, but the `.valueOf()` function then falls into
an infinite loop.  Assigning the same function to both `.toString()` and
`.valueOf()` not only eliminates this problem, but it also shrinks the code an
eensy weensy bit.

[UglifyJS2]: https://github.com/mishoo/UglifyJS2