class Rack::ConditionalGet

def fresh?(env, headers)

last request.
Return whether the response has not been modified since the 
def fresh?(env, headers)
  # if-none-match has priority over if-modified-since per RFC 7232
  if none_match = env['HTTP_IF_NONE_MATCH']
    etag_matches?(none_match, headers)
  elsif (modified_since = env['HTTP_IF_MODIFIED_SINCE']) && (modified_since = to_rfc2822(modified_since))
    modified_since?(modified_since, headers)
  end
end