A cache sends `If-None-Match` for a stored GET response and receives `304 Not Modified`. …

A cache sends `If-None-Match` for a stored GET response and receives `304 Not Modified`. What should it do?

1. Replace the stored representation with an empty body.
2. Keep the stored content and update permitted metadata from the 304 response.
3. Follow the `Location` field with POST.
4. Treat 304 as a server failure.