fix(index): add proxy-revalidate directive to cache-control

[Fdawgs]

must-revalidate covers both shared caches and private ones.

However, some incredibly old legacy proxies, which i seem to encounter a lot of in the public sector, only partially implement this functionality and require proxy-revalidate for clearing shared-caches as well. >:(

https://github.com/helmetjs/nocache already has this directive, probably for a similar reason.

Checklist

• Run npm test
• Documentation has been updated and adheres to the style described in CONTRIBUTING.md
• Commit message adheres to the Conventional commits style, following the @commitlint/config-conventional config

[Copilot]

Pull request overview

Adds the proxy-revalidate Cache-Control directive to better disable caching in legacy shared proxies that don’t fully honor must-revalidate, aligning behavior with similar middleware like helmetjs/nocache.

Changes:

• Update the plugin’s Cache-Control header value to include proxy-revalidate.
• Update the unit test expectation for the cache-control header.
• Update README header examples and rationale.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/index.js	Adds proxy-revalidate to the default cache-control response header.
test/index.test.js	Updates expected header value; adjusts JSDoc types for matchObject.
README.md	Documents the new Cache-Control value and explains why proxy-revalidate is included.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.