* lib/fileutils.rb: fix behavior when mkdir/mkdir_p accepted "/".
* test/fileutils/test_fileutils.rb: add test for above change.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41047 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
fix a typo

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41048 b2dd03c8-39d4-4d8f-98ff-823fe69b080e