Fixed #19508 -- Used uri_to_iri.

This is a PR for ticket #19508 for which another PR #2919 was opened by me a few days ago.
This PR implements `uri_to_iri` as per RFC and changes the `PATH_INFO` at the very time when `environ` is being set contrary to the previous one which corrected the received url from `environ`.
I have updated `ref/utils.txt`.
Regarding `ref/unicode.txt`, I wasn't sure whether we are to keep the docs in a similar way or probably we could improve upon the presentation.