Fallback to Digest::SHA512

`Gem::Package::TarWriter#add_file_signed` expects to fallback to
`Digest::SHA512`, and `digest.respond_to? :name` or not.
So lib/rubygems/security.rb should use same logic for
`Gem::Security::DIGEST_ALGORITHM` and `Gem::Security::DIGEST_NAME`.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@63851 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/rubygems/package/tar_writer.rb

@@ -187,8 +187,7 @@ class Gem::Package::TarWriter
         if digest.respond_to? :name then
           digest.name
         else
-          /::([^:]+)$/ =~ digest.class.name
+          digest.class.name[/::([^:]+)\z/, 1]
-          $1
         end
 
       digest_name == signer.digest_name

lib/rubygems/security.rb

@@ -344,14 +344,19 @@ module Gem::Security
       OpenSSL::Digest::SHA256
     elsif defined?(OpenSSL::Digest::SHA1) then
       OpenSSL::Digest::SHA1
+    else
+      require 'digest'
+      Digest::SHA512
     end
 
   ##
   # Used internally to select the signing digest from all computed digests
 
   DIGEST_NAME = # :nodoc:
-    if DIGEST_ALGORITHM then
+    if DIGEST_ALGORITHM.method_defined? :name then
       DIGEST_ALGORITHM.new.name
+    else
+      DIGEST_ALGORITHM.name[/::([^:]+)\z/, 1]
     end
 
   ##