module ActionDispatch::Http::Cache::Request

def fresh?(response)

supplied, both must match, or the request is not considered fresh.
If-Modified-Since and If-None-Match conditions. If both headers are
Check response freshness (Last-Modified and ETag) against request 
def fresh?(response)
  last_modified = if_modified_since
  etag          = if_none_match
  return false unless last_modified || etag
  success = true
  success &&= not_modified?(response.last_modified) if last_modified
  success &&= etag_matches?(response.etag) if etag
  success
end