feat(imagor): Expire Filter: Cache-Control response header

[headegg]

Previously, any request which used the expire() filter always set Cache-Control: "private, no-cache, no-store, must-revalidate" in the response.

As discussed here, it would be nice to be able to cache these responses in the browser, up to the expiry timestamp or imagor's configured cache ttl - whichever of the 2 is sooner.

This pull request makes the following changes:

• respect Cache-Control: private header, if present in the request and set the response Cache-Control header accordingly
• automatically set the private cache control request header when the expire() filter is used (assume resources with expiry are more sensitive and only to be cached privately)
• Set max-age in the Cache-Control response header to be either the seconds between now and the expire timestamp or imagor's configured ttl, whichever is the smaller.

[cshum]

Hi would you please merge from master branch again?

[headegg]

Hi @cshum. That's done. Thanks.

[cshum]

Thanks!