still needs test for urlunsplit() change

The pathsplit() function will work correctly on relative paths too so
don't say "absolute paths".  Improve comment for _get_redirect_url().

This avoids "manual parsing" of the Request-URI part of the request and
matches what _get_redirect_url() does.

Possible that someone could override this so a method is nicer.

Since pathsplit() doesn't seem like a generally useful public API,
remove it.  Instead, add a _request_path_split() method.  This ensures
that the redirect logic and the translate_path() method use the same
path parsing.