HTTPProxyServer should not send Content-Length header in successful responses to CONNECT requests

According to RFC7231,

A server MUST NOT send any Transfer-Encoding or Content-Length header fields in a 2xx (Successful) response to CONNECT.

However, I tried the example code on https://docs.ruby-lang.org/en/2.2.0/WEBrick/HTTPProxyServer.html

require 'webrick'
require 'webrick/httpproxy'

proxy = WEBrick::HTTPProxyServer.new Port: 8000

trap 'INT'  do proxy.shutdown end
trap 'TERM' do proxy.shutdown end

proxy.start

and sent a request through this proxy

$ ALL_PROXY=http://localhost:8000 curl -I https://www.google.com/

and it showed

HTTP/1.1 200 OK
Server: WEBrick/1.7.0 (Ruby/3.1.0/2021-12-25)
Date: Wed, 26 Jan 2022 03:48:33 GMT
Content-Length: 0
Connection: close

...

You can see the Content-Length header is there, and it does cause problems for some kinds of clients.

Start by running the WEBrick::HTTPProxyServer example and the provided curl command to reproduce the header. Then trace the HTTPProxyServer handling of CONNECT responses. Done means successful 2xx CONNECT responses no longer include Content-Length or Transfer-Encoding, consistent with RFC7231.