Cookie names are converted to lower case, causing cookie assertion failures

Describe the bug

Node.js version: 16, but reproduced with 20 and 24 too.

OS version: Ubuntu 24.04

Description: cookie names are converted to lower case.

Actual behavior

Given an app which sets a cookie Foo=bar:

app.get('/v1/myroute', (req, res) => {
  // Set a cookie with a mixed-case name
  res.cookie('Foo', 'bar');
  res.json({ message: 'Cookie set' }); 
});

When I write a test to expect the cookie Foo:

    const res = await request(app)
      .get('/v1/myroute/')
      .set('Accept', 'application/json')
      .expect(200)
      .expect(cookies.new({name: 'Foo', value: 'bar'}));

And I run npm test:

Error: expected: Foo cookie to be set

Then the test fails.

Expected behavior

The test passes.

Code to reproduce

See this code reproducer: https://github.com/caarmen/supertest-cookie-issue/tree/main

And a failing test PR: https://github.com/forwardemail/supertest/pull/880

Checklist

• I have searched through GitHub issues for similar issues.
• I have completely read through the README and documentation.
  • I read the cookies part in more detail, kind of skimmed through the rest of the README.
• I have tested my code with the latest version of Node.js and this package and confirmed it is still not working.