Reduce with no callback should reduce the array

If no callback is passed to `reduce`, it will now collapse a multi-dimensional array into a single array.

---

Sample usage:

```php
// Get all tags for the latest blog posts
$tags = Post::with('tags')->latest()->take(30)->get()->fetch('tags')->reduce()->unique();
```

Previously, the above would have had to be written like this:
```php
$tags = Post::with('tags')->latest()->take(30)->get()->fetch('tags');

$tags = $tags->reduce(function ($result, $tags)
{
    return array_merge($result, $tags);
}, []);

$tags = new Illuminate\Eloquent\Collection($tags)->unique();
```