Allow incrementing offsets in a collection.

I actually needed this for a project and was surprised that I had to use associative arrays with a collection. At the moment if you add an item to a collection without a key.

```php
$collection[] = 'Hello World';
```

The dump of the collection looks like this.

```
object Illuminate\Support\Collection (1) {
    protected items -> array(1) [
        '' => string (3) "Hello World"
    ]
}
```

This PR will allow it to behave like a normal array. Tests included.