public function getMetadata($key = null, $default = null)
    {
        if (null === $key) {
            return $this->metadata;
        }

        if (!is_scalar($key)) {
            throw new Exception\InvalidArgumentException('Non-scalar argument provided for key');
        }

        if (array_key_exists($key, $this->metadata)) {
            return $this->metadata[$key];
        }

        return $default;
    }