feature: add method arrayKeys with an additional support for ArrayObject

there's another alternative too:

```php
if (is_object($input)) {
    $array = get_object_vars($input);
   return array_keys($input);
}
```

And generally, all of this can be cascaded, for clarity of a single `return` statement:

```php
if (is_object($input)) {
    if ($input instanceof \ArrayObject) {
        ...
    } else {
        ...
    }
}

return is_array($input) ? array_keys($input) : false;
```