FR: http browser caching

[MattiJarvinen]

Htmlcache should set a few headers to enhance browser features.

If you have additional headers you'd like to see, please comment with a description, source/explanation/reason etc etc.

Headers:

• cache-control: no-cache, no store or Cache-Control: private with max-age
• expires?
• etag
• last-modified

Handle correctly:

• if-none-match
• if-modified-since

Sources:
https://jakearchibald.com/2016/caching-best-practices/
https://devcenter.heroku.com/articles/increasing-application-performance-with-http-cache-headers

Btw. is there a way to prevent headers & content being sent before onEndRequest? I was thinking if refreshing stale cache could be possible with etag & etc.