Respect Cache Header

[hottehead]

I scanned the docs and the code and I saw that the cache plugin does not respect cache header Expires and Cache-Control which may be sent from the upstream.

Is this correct? Is this on purpose?
If not I would implement this and create a PR.

[jpodwys]

You're correct, this repo does not respect cache headers. I'm open to supporting this (thank you for being willing to open a PR).

There is already a fork of this repo that adds cache header support. They submitted a PR in early 2018. At the time, I was weary of adding third-party libraries directly to this repo, so I raised that concern.

I'm now willing to reconsider merging their changes into this repo. It looks like they're currently 21 commits ahead and 6 commits behind, so there may be some conflicts, but probably nothing big.

Please have a look at that fork and let me know if it would meet your needs.

[hottehead]

I checked the PR. This meets my needs. I think the solution is well done and should be merged.

How about reopening the PR? The changes seem valid as is. And there are no more required changes on master that are not on the feature branch.

Using https://github.com/kornelski/http-cache-semantics#readme package seems valid even though it's third party, considering the complexity of cache policies.

[jpodwys]

Thank you for your thoughts! I'll reopen the PR and reach out to the fork owners.