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`.
Same as above:
```
path = uri_to_iri(path).encode(UTF8)
env['PATH_INFO'] = path if six.PY2 else path.decode(LATIN1)
```
Also comment is wrong, you don't latin-1 encode here, you latin-1 decode.