fix: support global "_Headers" in Undici 5.26.3

[joshkel]

Node.js's update to Undici 5.26.3 caused its Headers class to be called _Headers:

https://github.com/nodejs/node/pull/50153/files#diff-f516ab824a7722da938a4c7c851520d39731ddeb4f7198dff4e932c5d4f8fdf7

Fixes #72.

[joshkel]

Alternatively, would it be better to check instanceof if a global Headers class exists? Perhaps something like this?

    /**
     * @note Cannot necessarily check if the `init` is an instance of the
     * `Headers` because that class may not be defined in Node or jsdom.
     */
    if (
      ['Headers', 'HeadersPolyfill'].includes(init?.constructor.name) ||
      init instanceof Headers ||
      (typeof globalThis.Headers === 'function' && init instanceof globalThis.Headers)
    ) {

[kettanaito]

Thanks for fixing this, @joshkel!

[kettanaito]

We should also add a test for this, implicitly. Bumping the used Node.js version in the CI would be enough. I recommend adding a matrix of Node.js versions so we test both older versions and the recent ones that introduce this change.

Let me know if you need help with this.

[joshkel]

@kettanaito Done, I think. I have no real experience working with GitHub Actions, so I don't know if my approach is will work and is the best way of doing it.

[kettanaito]

You did it right 👌

[kettanaito]

This looks fantastic. Thank you, @joshkel!

[kettanaito]

Welcome to contributors, @joshkel 🎉

[kettanaito]

Released: v4.0.2 🎉

This has been released in v4.0.2!

• 📄 Release notes
• 📦 npm package

Make sure to always update to the latest version (npm i headers-polyfill@latest) to get the newest features and bug fixes.

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