Page Not Found on redirects containing spaces

I'm running into an issue where redirects aren't successful if the URL for the original destination page contains spaces (either raw spaces or encoded as a %20 entity). The problem appears to be that Drupal rewrites the destination URL before CustomError gets it, and any %20 entities are converted to raw spaces. Since CustomError uses the space character as a delimiter, I can't embed spaces in the URL mapping list and so the mapping never matches.

I've created a patch to take the original destination URL and replace spaces with %20 before checking for a match.