module ActionDispatch::Http::Cache::Request

def fresh?(response)

Reference: http://tools.ietf.org/html/rfc7232#section-6
`config.action_dispatch.strict_freshness`.
or both are considered equally. You can adjust the preference with
If both headers are supplied, based on configuration, either `ETag` is preferred over `Last-Modified`
`If-Modified-Since` and `If-None-Match` conditions.
Check response freshness (`Last-Modified` and `ETag`) against request 
def fresh?(response)
  if Request.strict_freshness
    if if_none_match
      etag_matches?(response.etag)
    elsif if_modified_since
      not_modified?(response.last_modified)
    else
      false
    end
  else
    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
end