Merge trunk revision: 39324

* lib/rubygems/installer.rb:  Use gsub instead of gsub! to avoid
  altering @bin_dir.  Fixes tests on windows.  [ruby-trunk - Bug #7885]


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@39343 b2dd03c8-39d4-4d8f-98ff-823fe69b080e