Avoid revalidating immutable HTTP responses (#1069)
## Summary
If you send a revalidation request to a resource that returns an
`immutable` directive, the server apparently returns a 200 instead of a
304? In other words, the server can ignore the revalidation request.
This PR adds handling on top of the HTTP cache semantics to respect
immutable resources, which is especially useful since all PyPI files are
immutable.
