* tool/downloader.rb (RubyGems.download): Don't download gem if the
  version is already downloaded. A gem file is versioned and
  it must be identical if the version is the same.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48494 b2dd03c8-39d4-4d8f-98ff-823fe69b080e