Decode user and password from env configured proxy

If someone sets an env variable defining a http_proxy, containing a
username / password with percent-encoded characters, then the resulting
base64 encoded auth header will be wrong.

For example, suppose a username is `Y\X` and the password is `R%S] ?X`.
Properly URL encoded the proxy url would be:

    http://Y%5CX:R%25S%5D%20%3FX@proxy.example:8000

The resulting proxy auth header should be: `WVxYOlIlU10gP1g=`, but the
getters defined by ruby StdLib `URI` return a username `Y%5CX` and
password `R%25S%5D%20%3FX`, resulting in `WSU1Q1g6UiUyNVMlNUQlMjAlM0ZY`.
As a result the proxy will deny the request.

Please note that this is my first contribution to the ruby ecosystem, to
standard lib especially and I am not a ruby developer.

Sorry for that and a happy and healthy 2021!

References:

- https://gitlab.com/gitlab-org/gitlab/-/issues/289836
- https://bugs.ruby-lang.org/projects/ruby-master/repository/trunk/revisions/58461
- https://bugs.ruby-lang.org/issues/17542

lib/net/http.rb

@@ -1180,7 +1180,9 @@ def proxy_port
     # The username of the proxy server, if one is configured.
     def proxy_user
       if ENVIRONMENT_VARIABLE_IS_MULTIUSER_SAFE && @proxy_from_env
-        proxy_uri&.user
+        require 'cgi/util'
+        user = proxy_uri&.user
+        CGI.unescape(user) if user
       else
         @proxy_user
       end
@@ -1189,7 +1191,9 @@ def proxy_user
     # The password of the proxy server, if one is configured.
     def proxy_pass
       if ENVIRONMENT_VARIABLE_IS_MULTIUSER_SAFE && @proxy_from_env
-        proxy_uri&.password
+        require 'cgi/util'
+        pass = proxy_uri&.password
+        CGI.unescape(pass) if pass
       else
         @proxy_pass
       end

test/net/http/test_http.rb

@@ -188,6 +188,23 @@ def test_proxy_eh_ENV_with_user
     end
   end
 
+  def test_proxy_eh_ENV_with_urlencoded_user
+    TestNetHTTPUtils.clean_http_proxy_env do
+      ENV['http_proxy'] = 'http://Y%5CX:R%25S%5D%20%3FX@proxy.example:8000'
+
+      http = Net::HTTP.new 'hostname.example'
+
+      assert_equal true, http.proxy?
+      if Net::HTTP::ENVIRONMENT_VARIABLE_IS_MULTIUSER_SAFE
+        assert_equal "Y\\X", http.proxy_user
+        assert_equal "R%S] ?X", http.proxy_pass
+      else
+        assert_nil http.proxy_user
+        assert_nil http.proxy_pass
+      end
+    end
+  end
+
   def test_proxy_eh_ENV_none_set
     TestNetHTTPUtils.clean_http_proxy_env do
       assert_equal false, Net::HTTP.new('hostname.example').proxy?