Selecting a response with ETags

[mnot]

In theory, a server can disambiguate multiple responses in cache by sending an ETag.

If we can get a cache into a state where it holds multiple valid, stale responses for a given request, we could test this.

Interaction with Vary might be interesting here.

If we can get cache into that state, we can also check to see if it sends multiple ETags in If-None-Match.