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 suggested this realiasing for tests only, I'd prefer a `if six.PY3 unquote_to_bytes() else unquote` in the actual function. It helps with comprehension.