Fix multi-header compatibility with Scrapy

[gg]

ImpersonateDownloadHandler._download_request() currently returns a scrapy.http.Response where multiple headers with the same key are concatenated into a single comma separated value. This means that for example, response.headers.getlist('Set-Cookie') returns a list with 1 item even if multiple Set-Cookie headers were present in the HTTP response.

To be compatible with Scrapy's built-in HTTP11DownloadHandler and H2DownloadHandler, response.headers.getlist(key) should return multiple values when multiple headers with the same key are present in the HTTP response.

To fix this, we can pass curl_cffi.requests.Headers.multi_items() to Scrapy's Headers() constructor.

[jxlil]

Thanks!