301 redirects (with default `redirect: 'follow'`) are not cached in v5

Before v5 (not quite released as I file this issue) all GETs got cached forever. In v5 we are fixing that (#100) which fixes the reported issue (#39) that 302s (temporary redirects) were being cached.

However, this introduces the new issue that 301s (permanent redirects) are now *not* being cached. That's because (assuming use have the default `redirect: 'follow'` in your fetch request) RESTDataSource only sees the response from the final request, not any intermediate request.

Fixing this would require us to implement redirect support ourselves which is probably not a great idea, so this may be more of a "documentation of an issue" than something that's likely to be fixed.

Start by reading the v5 caching change in issue #100 and the original 302 redirect report in issue #39, then inspect RESTDataSource's handling of the default `redirect: 'follow'` behavior. Determine whether the intended outcome is documentation of the 301 caching limitation or a design for redirect handling; completion is not defined in this issue.

Written by the indexing model from the issue text.