Fix: Add HttpStatusCodeLiteral 451

[jokj624]

All Submissions:

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you written unit tests?
• Have you written unit tests that cover the negative cases (i.e.: if bad data is submitted, does the library respond properly)?
• This PR is associated with an existing issue?

Closing issues

closes #1277

If this is a new feature submission:

• Has the issue had a maintainer respond to the issue and clarify that the feature is something that aligns with the goals and philosophy of the project?

Potential Problems With The Approach

Test plan

[github-actions]

Hello there jokj624 👋

Thank you and congrats 🎉 for opening your first PR on this project.✨

We will review the following PR soon! 👀

[WoH]

Afaik last time I was running into the discrepancies, Koa didn't support this.
https://github.com/koajs/koa/blob/master/docs/api/response.md

But the tests speak for themselves. LGTM, thanks!