Change type hint from 'string|ExternalReference' to 'mixed'

[Bloemendaal]

The Laravel In validation accepts Enums. However, the type of this method blocks me from using this. Also the method that is being called expects mixed :)

Laravels implementation:

    public function __toString()
    {
        $values = array_map(function ($value) {
            $value = enum_value($value);

            return '"'.str_replace('"', '""', $value).'"';
        }, $this->values);

        return $this->rule.':'.implode(',', $values);
    }