feat: add support web WHATWG

[kravorkid]

Feat to handle WHATWG #1777 I'm open to any suggestions

Checklist

• [x ] I have ensured my pull request is not behind the main or master branch of the original repository.
• [x ] I have rebased all commits where necessary so that reviewing this pull request can be done without having to merge it first.
• [x ] I have written a commit message that passes commitlint linting.
• [x ] I have ensured that my code changes pass linting tests.
• [x ] I have ensured that my code changes pass unit tests.
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[fengmk2]

Should convert the body data type on respond https://github.com/koajs/koa/blob/master/lib/application.js#L302

A full http request test needs to be added to ensure that the response content is the expected result. e.g.: https://github.com/koajs/koa/blob/master/__tests__/application/respond.js#L175

[kravorkid]

@fengmk2 i've converted body data type and added some tests. I'm not working on that kind of data on a daily basis so i'm quite unsure if i'm testing the right thing & every corner case Just tell me if something is missing 🙂