Fix a Pathname#relative_path_from crash on Windows.

Pathname#relative_path_from uses String#casecmp to compare strings on
Windows. This can return nil for strings with different encodings, and
the code previously assumed that it always returned a Fixnum.

For example:

```ruby
Pathname.new("foö".encode("UTF-8")).relative_path_from(
    Pathname.new("bär".encode("ISO-8859-1")))
```