Number.prototype.toFixed() against big numbers doesn't have enough precision.

I'm using Microsoft Edge 42.17134.1.0.

I noticed some bug in `Number.prototype.toFixed()`. For example, following code shows unexpected result on Microsoft Edge.

```javascript
x = 1125899906842623.9;
console.log(x.toFixed(2)); // Edge: 1125899906842623.90, Chrome: 1125899906842623.88
```

Microsoft Edge (and also IE 11) returns `"1125899906842623.90"`. However, Chrome and other browsers return `"1125899906842623.88"`.

ECMAScript specification requires that `Number.prototype.toFixed()` must return mathematically closest decimal expression to `x`. The exact value of `x` is `1125899906842623.875`, so the correct return value seems to be `"1125899906842623.88"`.