Parse response before checking length

If the fetch response is not a list but rather a list wrapped in an object, like

```js
{
   data: [ blah, blah, blah ]
}
```

then it needs to be parsed before calling `.length` on it to decide whether there are more pages to load. This is exactly what Backbone's `parse` function is used for. The default implementation of `parse` is to just pass the response through, so there's no need to check if it's there before calling it.

I don't think what's sent to `success` and `error` should be changed because people probably already depend on it working a certain way. It's less likely that anyone depends on `fetchSuccess`'s broken behavior in `strict: false` mode.