deprecate safe_conversion parameter of iri_to_uri

[davidism]

iri_to_uri has a safe_conversion parameter that returns the URL unchanged if it's only ASCII without spaces. This gets used in redirect and Response.get_wsgi_headers. The reason it exists is because urlunsplit sometimes skips adding // after scheme:. The two examples given were magnet:, which doesn't seem to demonstrate the issue, and itms-services:, which has no public spec and doesn't even appear to be an actively used Apple URL scheme.

If it turns out that this somehow breaks a real use case, I think the best course of action is to make it possible to disable the normalization in Response.get_wsgi_headers. Unfortunately, there's already an autocorrect_location_header attribute that means "make the URL absolute". We could try to think of another name, or figure out a way to repurpose the existing attribute name. Or better yet, get the issue fixed in urlunsplit instead.

Was going to write this as a PR, but accidentally merged instead of pushed the PR, so it's already in c3b26d0.