express.static - add support for cache-control: immutable

[techandsoftware]

On static content with versioned URLs, add a flag to options on express.static that generates a cache-control header with immutable, e.g.:
Cache-Control: max-age=365000000, immutable
This reduces unnecessary requests in supporting clients.

This is supported by Firefox 49 and used by Facebook:
https://hacks.mozilla.org/2017/01/using-immutable-caching-to-speed-up-the-web/
https://bitsup.blogspot.co.uk/2016/05/cache-control-immutable.html

Supported in Chrome 54
https://bugs.chromium.org/p/chromium/issues/detail?id=611416

Webkit support seems to be added:
https://bugs.webkit.org/show_bug.cgi?id=167497

[dougwilson]

Sounds reasonable :) One point missed above is that Chrome does not support this, as you can read through in that Chromium issue (they implement something similar to cache-control: immutable using heuristics instead of looking at a header value at all).

Thinking how we could easily add this to our API, what about having it where if you set maxAge: Infinity it would add the immutable tag in addition to the 1 year age it does today?

[techandsoftware]

Ok, I confess I didn't read all the comments. The maxAge: Infinity suggestion sounds good.

[dougwilson]

This landed in the upstream send dependency, so will be part of the Express.js 4.16 release.