public function setHeader(string $key, string $value): void
    {
        $this->buildStructuredHeaders();
        $lkey = strtolower($key);
        if (isset($this->structuredHeaders[$lkey])) {
            $this->structuredHeaders[$lkey]['value'] = $value;
        } else {
            $newHeader = array('key' => $key, 'value' => $value);
            $this->structuredHeaders[$lkey] = $newHeader;
        }
        $this->storeStructuredHeader();
    }