Inconsistent handling of casing in header names (CORS-safelisted response header)

[bogeychan]

What version of Bun is running?

1.1.29

What platform is your computer?

WSL Ubuntu

What steps can reproduce the bug?

Bun.serve({
  fetch() {
    return new Response("why is content-type lower case? :(");
  },
  port: 8080,
});

Bun.serve({
  fetch() {
    return Response.json({ yay: "content-type header is upper case :)" });
  },
  port: 8080,
});

What is the expected behavior?

All CORS-safelisted response headers should be upper case by default

What do you see instead?

Content-Type in return new Response("why is content-type lower case? :("); is lower case

Additional information

for more details: elysiajs/elysia-cors#48 (comment)