When a request contains an ETag value in its If-None-Match header, the backend
may send a request ID (from Rack) that does not correspond to a value in Peek's
Redis cache (because we aborted the Rails processing in our ETag middleware).

Because a cached response (304) has to replace the headers with those from the
previous 200 - https://tools.ietf.org/html/rfc7234#section-4.3.4 - we add a
custom header that will only be present in cache hits, that can tell the
frontend to ignore these.