downloader.rb: integrate with download_if_modified_since

* tool/downloader.rb (Downloader.download): integrate with
  download_if_modified_since and allow to use `since` parameter if it
  is not true/false.  also set last-modified time and permission.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47693 b2dd03c8-39d4-4d8f-98ff-823fe69b080e