[bug] Setting cookies does not take effect

## Describe the bug

**Node.js version:**
v16.20.2
**OS version:**
macOS 14.2.1
**Description:**

I use `set('Cookie',[xxx])`, but the cookie was not added to the request header
```
const request = require('supertest');
const server = require('./server.js');

const agent = request.agent(server);

const postRes = await agent.post('/verify').set('Cookie', [`csrf-token=xxxxx`]);
```

## Actual behavior

Cookie is not added to the request header

## Expected behavior

Cookies can be added to request headers

## Code to reproduce

My code is as follows，The first time I set cookie, it can be added to the request header correctly.But the second time I set cookie, it was not added to the request header.
```
const request = require('supertest');
const server = require('./server.js');

const agent = request.agent(server);
const postRes = await agent.post('/verify').set('Cookie', [`csrf-token=xxxxx`]);
const postRes1 = await agent.post('/verify').set('Cookie', [`csrf-token=xxxxx`]);
```
I briefly checked this problem, It's because my server returned the `set-cookie` response header. Then the second request will only bring the cookie returned by the first request, but not the cookie I set.like this
```
Cookie: token=xxx
```

I hope that both the cookies returned by the first request and the cookies I set are added to the response headers correctly. like this

```
Cookie: csrf-token=xxxxx;token=xxx
```

I checked the source code

https://github.com/ladjs/superagent/blob/1c8338b2e0a3b8f604d08acc7f3cbe305be1e571/src/node/index.js#L856


`hasOwn(this._header, 'cookie')` return `false`, because of my `set('Cookie',[xxx])` it will be added as `header['Cookie']=[xxx]`,instead of `header['cookie']=xxx`

If I write like this, it will add the cookie to the header correctly
```
const postRes = await agent.post('/verify').set('Cookie', [`csrf-token=xxxxx`]);
const postRes1 = await agent.post('/verify').set('cookie', `csrf-token=xxxxx`);
```

I don't know if this is a bug or if it's by design

## Checklist

- [x] I have searched through GitHub issues for similar issues.
- [x] I have completely read through the README and documentation.
- [x] I have tested my code with the latest version of Node.js and this package and confirmed it is still not working.