Here you are accessing `data['date_range']` when `data` is the JSON data passed to the `ReportConfig` constructor.

This doesn't seem right, because

```javascript
var foo = new ReportConfig({date_range: null});
foo.date_range("not null");
foo.have_date_range();  // false, but should be true
```

To fix this it seems like you could eliminate `have_date_range()` and just do in the constructor

```javascript
self.date_range = ko.observable(data['date_range'])
```

But wait!  That is already achieved by

```javascript
var self = ko.mapping.fromJS(data, {
        'copy': ['filters']
    });
```

So I'm unclear why this change is necessary.  Doesn't `self.date_range()` work as an equivalent for `self.have_date_range()` already?