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`.
We need idempotent functions that works reliably between python 2 and python 3. Then if the caller has specific needs, they can take care of their own edge cases. Whatever goes in `utils/encoding.py` should be considered "library" grade, just like werkzeug.