public function header(array $headers) {

        $formattedHeaders = [];

        $combinedHeaders = array_change_key_case(array_merge($this->header, $headers));

        foreach ($combinedHeaders as $key => $val)

            $formattedHeaders[] = trim(strtolower($key)) . ':' . $val;

        $this->header = $formattedHeaders;

        return $this;

    }