set expires header corresponding to maxAge

[RedSparr0w]

closes #1650
currently when using ?maxAge= the cache-control header is set correctly but the expires header is always set to the current time.

This PR:

Cache-Control: max-age=3600
Date: Mon, 16 Apr 2018 21:57:47 GMT
Expires: Mon, 16 Apr 2018 22:57:47 GMT

Current:

cache-control: max-age=3600
date: Mon, 16 Apr 2018 21:55:33 GMT
expires: Mon, 16 Apr 2018 21:55:33 GMT

[shields-ci]

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[platan]

Looks good for me. Would you like to add some tests?

[RedSparr0w]

Probably a good idea, although i'm really not sure how to test the headers.

---

Also according to RFC 2616 on W3

The max-age directive takes priority over Expires, so if max-age is present in a response, the calculation is simply:
    freshness_lifetime = max_age_value
Otherwise, if Expires is present in the response, the calculation is:
    freshness_lifetime = expires_value - date_value

So this PR may actually be pointless?

[RedSparr0w]

Have added some tests if you could please give it another look over @platan

[ale5000-git]

So this PR may actually be pointless?

It may be pointless in a perfect world but different implementations take different approach, there will always be someone not following the spec or maybe also not supporting Cache-Control but supporting Expires.

[ale5000-git]