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`.
If we mention `uri_to_iri()` here we need to document it. Dunno if we should make it a public API? If we do then the docs for `iri_to_uri()` can be used as guidance.

cc @timgraham, @aaugustin.