preserve invalid itms-services url scheme

[davidism]

Partially revert #2609. The iri_to_uri safe_conversion parameter is still deprecated. redirect no longer calls iri_to_uri, the only time the check is done is in Response.to_wsgi_headers. In 2.4, that should become a flag like response.process_location or normalize_location so that the exception and split aren't needed.

fixes #2691