Multi select box: 'old' value is not fetched correctly if input name is array

The following multi select box will not have its old input retrieved because its name is given in array syntax.

```php
Form::select('grades[]')->withAttributes(['multiple' => 'multiple']);
```

Workaround:

```php
Form::select('grades[]')->withAttributes(['multiple' => 'multiple'])->withSelected(old('grades'));
```