return invalid value from File.dirname on VisualStudio

mruby with VisualStudio:

``` ruby
File.dirname("")     #=> ""
File.dirname("a")    #=> ""
File.dirname("/a")   #=> "/"
File.dirname("a/b")  #=> "a/"
File.dirname("/a/b") #=> "/a/"
```

Ruby 2.1:

``` ruby
File.dirname("")     #=> "."
File.dirname("a")    #=> "."
File.dirname("/a")   #=> "/"
File.dirname("a/b")  #=> "a"
File.dirname("/a/b") #=> "/a"
```
