LibWeb/Fetch: Add Set-Cookie as a forbidden header name

See:
 - https://github.com/whatwg/fetch/commit/50d77e6
 - http://wpt.live/fetch/api/request/request-headers.any.html

Userland/Libraries/LibWeb/Fetch/Infrastructure/HTTP/Headers.cpp

@@ -702,6 +702,7 @@ bool is_forbidden_request_header(Header const& header)
             "Keep-Alive"sv,
             "Origin"sv,
             "Referer"sv,
+            "Set-Cookie"sv,
             "TE"sv,
             "Trailer"sv,
             "Transfer-Encoding"sv,