HTTPS proxy is only enabled if the proxied connection is also SSL

irb(main):001> require 'net/http'
=> true
irb(main):002> n = Net::HTTP.new('192.0.2.10', 80, '127.0.0.1', 5000, nil, nil, nil, true)
=> #<Net::HTTP 192.0.2.10:80 open=false>
irb(main):003> n.instance_variable_get(:@proxy_use_ssl)
=> true
irb(main):004> n.start
=> #<Net::HTTP 192.0.2.10:80 open=true>
irb(main):005> n.get('/')

Data sent to the listener at 127.0.0.1:5000:

GET http://192.0.2.10/ HTTP/1.1
Accept-Encoding: gzip;q=1.0,deflate;q=0.6,identity;q=0.3
Accept: */*
User-Agent: Ruby
Host: 192.0.2.10

This is due to the proxy SSL socket setup being inside a if use_ssl? block.

https://github.com/ruby/net-http/blob/master/lib/net/http.rb#L1667-L1674

Transporting clear HTTP over a HTTPS proxy is a valid configuration, the library should support this.

Start in lib/net/http.rb around lines 1667-1674 and reproduce the reported Net::HTTP configuration with a clear HTTP target and HTTPS proxy. Trace the proxy socket setup and verify that clear HTTP requests can be transported through an HTTPS proxy without changing the target connection. Confirm the behavior with a focused regression test using the shown configuration.