`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