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`.
Why can't we just return unicode (i.e. decoded bytestring, so `str` on PY3 and `unicode` on PY2)?

In that case the signature would be `uri_to_iri(uri, encoding='utf-8')` just like werkzeug.