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

[ag-TJNII]

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.