[node] Correct type of incoming http2 ':status' header

[Flarna]

• Use a meaningful title for the pull request. Include the name of the package modified.

• Test the change in your own code. (Compile and run.)

• Add or edit tests to reflect the change. (Run with npm test.)

• Follow the advice from the readme.

• Avoid common mistakes.

• Run npm run lint package-name (or tsc if no tslint.json is present).

• Provide a URL to documentation or source code which provides context for the suggested changes:
  doc: Improve doc for Http2 headers object nodejs/node#21296
  https://github.com/nodejs/node/blob/1442cfef73c0a7157b1552658529ae9ff9154de9/lib/internal/http2/util.js#L508-L509

• Increase the version number in the header if appropriate.

• If you are making substantial changes, consider adding a tslint.json containing { "extends": "dtslint/dt.json" }.

[typescript-bot]

@Flarna Thank you for submitting this PR!

🔔 @parambirs @tellnes @WilcoBakker @octo-sniffle @smac89 @mwiktorczyk @wwwy3y3 @DeividasBakanas @kjin @alvis @eps1lon @Hannes-Magnusson-CK @jkomyno @ajafff @hoo29 @n-e @BrunoScheufler @mohsen1 @KSXGitHub @a-tarasyuk @islishude - please review this PR in the next few days. Be sure to explicitly select Approve or Request Changes in the GitHub UI so I know what's going on.

If no reviewer appears after a week, a DefinitelyTyped maintainer will review the PR instead.

[typescript-bot]

A definition owner has approved this PR ⭐️. A maintainer will merge this PR shortly. If it shouldn't be merged yet, please leave a comment saying so and we'll wait. Thank you for your contribution to DefinitelyTyped!