cubicweb #11466875 304 responses are missing headers [validation pending]

When generating a 304 response, we should be sending the same ETag (and others) response header we would in a 200 response. Instead, we're looking for ETag in the request headers, which makes no sense.

prioritynormal
typebug
done in3.22.3
load1.000
load left0.000
closed by#eb88fdfd3740 [web] fix headers on 304 responses (closes #11466875), #5a19524e59e1 [web] fix headers on 304 responses (closes #11466875)
patch[web] fix headers on 304 responses [applied]