[rubygems/rubygems] Provide distinguished name which will be correctly parsed.

It seems that since ruby openssl 2.1.0 [[1]], the distinguished name
submitted to `OpenSSL::X509::Name.parse` is not correctly parsed if it
does not contain the first slash:

~~~
$ ruby -v
ruby 3.0.2p107 (2021-07-07 revision 0db68f0233) [x86_64-linux]

$ gem list | grep openssl
openssl (default: 2.2.0)

$ irb -r openssl
irb(main):001:0> OpenSSL::X509::Name.parse("CN=nobody/DC=example").to_s(OpenSSL::X509::Name::ONELINE)
=> "CN = nobody/DC=example"
irb(main):002:0> OpenSSL::X509::Name.parse("/CN=nobody/DC=example").to_s(OpenSSL::X509::Name::ONELINE)
=> "CN = nobody, DC = example"
~~~

Instead, use `OpenSSL::X509::Name.new` directly as suggested by upstream
maintainer.

[1]: 19c67cd10c

https://github.com/rubygems/rubygems/commit/09ca0c2dae

Co-authored-by: Kazuki Yamaguchi <k@rhe.jp>

lib/rubygems/security.rb

@@ -510,9 +510,10 @@ module Gem::Security
 
     dcs = dcs.split '.'
 
-    name = "CN=#{cn}/#{dcs.map {|dc| "DC=#{dc}" }.join '/'}"
-
-    OpenSSL::X509::Name.parse name
+    OpenSSL::X509::Name.new([
+      ["CN", cn],
+      *dcs.map {|dc| ["DC", dc] },
+    ])
   end
 
   ##

test/rubygems/test_gem_security.rb

@@ -196,7 +196,7 @@ class TestGemSecurity < Gem::TestCase
 
   def test_class_sign
     issuer = PUBLIC_CERT.subject
-    signee = OpenSSL::X509::Name.parse "/CN=signee/DC=example"
+    signee = OpenSSL::X509::Name.new([["CN", "signee"], ["DC", "example"]])
 
     key  = PRIVATE_KEY
     cert = OpenSSL::X509::Certificate.new