Makes Schema::getColumnListing('table_name') work irrespective of the PDO fetch mode

When PDO fetch mode is set to return an array, Laravel currently fails with the following error

```
Notice: Trying to get property of non-object in Illuminate/Database/Query/Processors/MySqlProcessor.php on line 13
```

This pull request checks for type and gets the column names accordingly!

Closes #4727
Why not just ?
```php
return array_map(function ($r) { return data_get($r, 'column_name'); }, $results);
```