pathname.rb: fix a Pathname#relative_path_from crash on

* ext/pathname/lib/pathname.rb (SAME_PATHS):
  Pathname#relative_path_from uses String#casecmp to compare strings
  on case-insensitive filesystem platforms (e.g., Windows). This can
  return nil for strings with different encodings, and the code
  previously assumed that it always returned a Fixnum.  [Fix GH-713]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47591 b2dd03c8-39d4-4d8f-98ff-823fe69b080e