merges r20496 from trunk into ruby_1_9_1.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_9_1@20510 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
merges r20496 from trunk into ruby_1_9_1.
this method private. a patch from okkez in [ruby-dev:37245]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_1_9_1@20510 b2dd03c8-39d4-4d8f-98ff-823fe69b080e