Regression in Collection#fetch() with options.add = false

Fixes `Uncaught TypeError: Cannot read property 'id' of undefined` error being thrown by the code below:

```coffee
@collection.fetch add: no, success: (collection, data) =>
  @collection.reset _.shuffle(data)
```

There was no such problem in v0.6.3 which I've been using before, so it's either regression or intentional change and I'm betting on the former. If so, I hope this change will mitigate the issue, otherwise I'd curious to know why such change was introduced.