Use Cache-Control max-age instead of Expires header

[teohhanhui]

No description provided.

[teohhanhui]

Try to fix jimmynicol/image-resizer#27

[stevenscg]

@teohhanhui Did you look at possibly leaving the Vary header in place?

gzip vs non-gzip requests, for example, could be an issue as described here:
https://www.fastly.com/blog/best-practices-for-using-the-vary-header:

The bigger problem is when the first request for an object comes from a browser that does do compression, and we end up with the compressed version in our cache. Now, when a browser comes along that doesn't understand compression, it gets the compressed version and has no idea what to do other than display gibberish.

[teohhanhui]

@stevenscg Adding Vary: Accept-Encoding is the responsibility of whatever is handling the compression. For example, nginx and Varnish already correctly sets that header when compression is enabled. If you're doing the compression in your node app, please be sure to set that header yourself.