Add support for more HTTP cache controls

[dasch]

Adds support for the stale-while-revalidate and stale-if-error extensions to the Cache-Control response header.

From https://tools.ietf.org/html/rfc5861:

The stale-if-error HTTP Cache-Control extension allows a cache to
return a stale response when an error -- e.g., a 500 Internal Server
Error, a network segment, or DNS failure -- is encountered, rather
than returning a "hard" error. This improves availability.

The stale-while-revalidate HTTP Cache-Control extension allows a
cache to immediately return a stale response while it revalidates it
in the background, thereby hiding latency (both in the network and on
the server) from clients.

These are useful, fully standardized parts of the HTTP protocol with widespread support among CDN vendors. Supporting them will make it easier to utilize reverse proxies and CDNs from Rails without resorting to hacks.

The expires_in method doesn't seem to document the various options – is there a specific place I should document this?

[dasch]

r? @eileencodes

[eileencodes]

Thanks!

[dasch]

❤️!

[mikhailov]

this PR is extremely useful to cover use cases like one described by Cloudflare:

Cache an asset and serve the asset while it is being revalidated
Cache-Control: max-age=600, stale-while-revalidate=30

Indicates that it is fresh for 600 seconds, and it may continue to be served stale for up to an additional 30 seconds to parallel requests for the same resource while the initial synchronous revalidation is attempted.