Endpoint Response breaks with "content-type: application/json"

Describe the bug

Using the new api described in #3384, when I create an API endpoint returning a Response, it breaks in Cloudflare Workers if I set content-type: application/json as a Response header.

I believe the reason is because it applies JSON.stringify to the body of the response. However the body of the response being a ReadableStream, it breaks returning an object { locked: false }.

Reproduction

I believe the culprit is found here: https://github.com/sveltejs/kit/blob/ee906a3b2b103e4523eb1b6bb13f04d03eb35238/packages/kit/src/runtime/server/endpoint.js#L86

Normally the control is_pojo(body) shouldn’t allow the code to reach here, however the control doesn’t work correctly in Cloudflare Workers. More specifically it fails here: https://github.com/sveltejs/kit/blob/ee906a3b2b103e4523eb1b6bb13f04d03eb35238/packages/kit/src/runtime/server/endpoint.js#L119

In Cloudflare body[Symbol.toStringTag] returns undefined. Replacing the control with body instanceof ReadableStream works in Cloudflare Workers.

Logs

No response

System Info

Binaries:
    Node: 16.10.0 - ~/.volta/tools/image/node/16.10.0/bin/node
    Yarn: 3.1.0 - ~/.volta/tools/image/yarn/1.22.17/bin/yarn
    npm: 8.3.1 - ~/.volta/tools/image/npm/8.3.1/bin/npm
  npmPackages:
    @sveltejs/adapter-cloudflare: ^1.0.0-next.8 => 1.0.0-next.8 
    @sveltejs/kit: ^1.0.0-next.239 => 1.0.0-next.239 
    svelte: ^3.46.2 => 3.46.2 
    vite: ^2.7.12 => 2.7.12

Severity

serious, but I can work around it

Additional Information

No response

[vicky1999]

okay. Thanks. I tried to run the test cases. all the test cases were passing

[bluwy]

@vicky1999 Not sure how easy it is to reproduce this, since it happens on cloudflare specifically, but as long as the test pass with your changes. I think that’s good enough.