An unknown code should be handled the same as the x00 code of this
group, but for redirections we used to treat 300 (and a few others)
as an error while unknown codes were considered redirections.
Instead we check now explicitly for the redirection codes we support for
redirecting (and add the 308 defined in RFC 7538) to avoid future
problems if new 3xx codes are added expecting certain behaviours.
Potentially strange would have been e.g. "305 Use Proxy" sending a
Location for the proxy to use – which wouldn't have worked and resulted
in an error anyhow, but probably confused users in the process.