[Proxy] scrubbed set-cookie header; set-cookie for wrong domain

Hello, I am building a proxy server, then I got error below

`scrubbed set-cookie header; set-cookie for wrong domain`

code from here: https://deno.land/x/oak_http_proxy

Start with the proxy code at https://deno.land/x/oak_http_proxy and reproduce the reported “scrubbed set-cookie header; set-cookie for wrong domain” error using the proxy setup described. Trace how the upstream cookie domain is checked and identify the request and cookie conditions that trigger the message; done means the behavior is understood and the issue has a reproducible, actionable diagnosis.

Written by the indexing model from the issue text.