fs::mkdir returns OtherIoError instead of PathAlreadyExists

[retep998]

When creating a directory on windows using fs::mkdir, if the directory already exists it returns an error where kind is OtherIoError instead of PathAlreadyExists as I would expect.

[steveklabnik]

fs::create_dir now returns a ErrorKind::PathAlreadyExists, so this has been fixed. woo!