Ruby

Add tests for File.extname

• file.c (rb_file_s_extname): [DOC] add an example.
• test/ruby/test_path.rb (test_extname): add tests. [Fix GH-978]
  • path starts with dot ('.a.rb')
  • path includes dir name ('a/b/d/test.rb')
  • path includes dir name and dir name starts with dot
    ('.a/b/d/test.rb')

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@51359 b2dd03c8-39d4-4d8f-98ff-823fe69b080e