* lib/fileutils.rb: enable to remove with non-owner directory.
  [ruby-dev:45976] [Bug #6756]
* test/fileutils/test_fileutils.rb: add testcase for #6756.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47150 b2dd03c8-39d4-4d8f-98ff-823fe69b080e