The option --unicode-beta for tool/downloader.rb introduced in
r66448 must not be passed as an option to actual download machinery.
Thanks to MSP-Greg (Greg L.) for bug report and patch.
This closes issue #15434.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@66449 b2dd03c8-39d4-4d8f-98ff-823fe69b080e