[4.2] Fix groupBy() bug

As described here https://github.com/laravel/framework/pull/4276#issuecomment-45563986 by @JosephSilber
I think not, but, `is_callable()` accepts this context:

```php
class a{
   public function b(){}
}

$a = new a();
var_dump(is_callable($a, 'b')); // true
```