Expires: and Cache-control: max-age=NNN
	(1) Origin servers may send either or both of
		Cache-control: max-age=N
	on responses
	(2) If both are sent, an HTTP/1.1 implementation SHOULD
	obey the Cache-control: max-age=N directive, and so SHOULD ignore
	the Expires field value.
	(2b) but HTTP/1.0 implementations will ignore it, so servers
	SHOULD NOT depend on a cache obeying that directive.

	The behavior of max-age in a response should be equivalent to
	what would occur if this response had an Expires: field
	specifying the same number of seconds since its Date: field,
	but max-age overrides an explicit Expires: value.
Not agreed: When neither the Expires: header nor Cache-control: max-age is sent, should the rules determining what assumptions a cache can make about the expiration date depend on whether the origin server is HTTP/1.0 or HTTP/1.1?