dir.c: glob cases on case-insensitive system

* dir.c (glob_helper): return the filename with actual cases on
  the filesystem if it is case-insensitive.  [ruby-core:42469]
  [Feature #5994]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44796 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-02-03

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44797 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* error.c: [DOC] Exception#cause may return nil. [ci skip]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@44798 b2dd03c8-39d4-4d8f-98ff-823fe69b080e