tool/downloader.rb: added Downloader.download_if_modified_since
to reduce downloads of large files that change only rarely.
[ruby-core:65164] [CommonRuby - Feature #10084]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47691 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* remove trailing spaces.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47692 b2dd03c8-39d4-4d8f-98ff-823fe69b080e