keep relative redirect locations by default

[davidism]

When Response.autocorrect_location_header was added in 2011 0ec643b, it was documented as "correct the location header to be RFC conformant". It was referring to RFC 2616. That was superseded by RFC 7231 in 2014, which allows relative URLs. MDN lists all browsers as compliant with this. Switch autocorrect_location_header to be disabled by default.

[davidism]

I've confirmed that Firefox, Chrome, Edge, urllib.request.urlopen, urllib.request.FancyURLOpener, requests.get, httpx.get, curl -L, wget, and aria2c all follow relative redirect correctly.