Collection method unique() does not differentiate the strings '0' and '00'

With following example I expected to get both arrays back. Only got the first one with 'prefix' => '0'

```
$collect = collect(
[
    [
        'prefix' => '0',
        'trunk'  => 'A'
    ],[
        'prefix' => '00',
        'trunk'  => 'B'
    ]
]);

var_dump($collect->unique('prefix'));
```

I expected that because they are both Strings and therefore should be different.

Nice regards
Kristof
