`fs::rename` behavior differs on windows

This program will succeed on unix but fail to succeed on windows. The unix behavior is surprising to me!

``` rust
use std::io::{fs, UserRWX};

fn main() {
    fs::mkdir(&Path::new("test"), UserRWX).unwrap();
    fs::mkdir(&Path::new("test2"), UserRWX).unwrap();
    fs::rename(&Path::new("test"), &Path::new("test2")).unwrap();
}
```
