Regression from 04d5a556. Keying the resize cache on identity meant the
disc response no longer carried a content hash, and the full-size branch
set no ETag either — so WriteImageHeaders fell back to the empty hash
and emitted `ETag: ""` for every full-size disc image. Since ifNoneMatch
compares the unquoted value, a client echoing that back matched, and got
a 304 even after the image was replaced.
The full-size branch now carries the same identity validator the sized
branch uses, so it moves when the folder's images change.
WriteImageHeaders is hardened against the class as well: an empty
validator is no validator, so it is neither emitted nor matched.
Reported by Codex on #5847.
6bb3e98e4c