protected function prepareHeaders(array $headers)
    {
        return array_merge($this->getDefaultHeader(), array_filter($headers, function ($key) {
            return !in_array(strtolower($key), self::$excludeHeaders);
        }, ARRAY_FILTER_USE_KEY), ['content-length' => 0]);
    }
protected function prepareHeaders(array $headers)
    {
        return array_merge($this->getDefaultHeader(), array_filter($headers, function ($key) {
            return !in_array(strtolower($key), self::$excludeHeaders);
        }, ARRAY_FILTER_USE_KEY), ['content-length' => 0]);
    }
{@inheritdoc}