curl: handle multiple Content-Type headers

[samford]

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#curl_http_content_headers_and_checksum contains code that works with a Content-Type header in a response but it expects there to always be only one header in the response. This is normally a reasonable assumption but we've come across a server that is giving a response with multiple Content-Type headers (Homebrew/homebrew-cask#189072), so this produces an error and causes brew audit to fail when checking the URL.

This works around the issue by naively using the last Content-Type header in the response when there's more than one. It's not something that should normally occur but this will handle the situation when it does.

[ZhongRuoyu]

This seems to be forbidden by the standard, but things do happen...

[MikeMcQuaid]

Thanks @samford!