Collection merge does not work without an overridden getKey() method

I realised that if you want to merge two collections like this:

```
$collection1->merge($collection2)
```

You will get an exception saying that you haven't implemented a **getKey()** method. After som digging around, I found that, as long as you implement this method on the model and return something unique for each model instance, the merge will work.

I guess my tip is that the getKey() method is implemented in the **Model.php** file and that the developers using this package can provide an **id** attribute which is unique for all model instances :)