[fix] content-type with "header=" breaks all header matchers

## Describe the bug

**Node.js version:** v24.10.0

**OS version:** macOS 15.7.3

**Description:**

If a response contains a `Content-Type` header with `header=(anything)`, header matching breaks for _all_ headers. This is relevant to `text/csv` responses, which can contain `header=present` or `header=absent`.

## Actual behavior

Expectations fail with `expected "..." header field` (see full output below)

## Expected behavior

Other headers are not affected by `content-type` parameters.

## Code to reproduce

```js
import request from 'supertest';
import { createServer } from 'node:http';

const s = createServer((_, res) => {
res.setHeader('foo', 'a');
res.setHeader('content-type', 'text/csv; header=a');
res.end();
});
await new Promise((resolve) => s.listen(0, '127.0.0.1', resolve));

await request(s).get('/').expect(200).expect('foo', 'a');
```

> ```
> Error: expected "foo" header field
> at file://[...]/experiment.js:11:39
> at process.processTicksAndRejections (node:internal/process/task_queues:105:5)
> ----
> at Test._assertHeader ([...]/node_modules/supertest/lib/test.js:281:47)
> at [...]/node_modules/supertest/lib/test.js:365:13
> at Test._assertFunction ([...]/node_modules/supertest/lib/test.js:342:13)
> at Test.assert ([...]/node_modules/supertest/lib/test.js:195:23)
> at localAssert ([...]/node_modules/supertest/lib/test.js:138:14)
> at [...]/node_modules/supertest/lib/test.js:156:7
> at Request.callback ([...]/node_modules/superagent/lib/node/index.js:847:12)
> at IncomingMessage. ([...]/node_modules/superagent/lib/node/index.js:1102:18)
> at IncomingMessage.emit (node:events:520:35)
> at endReadableNT (node:internal/streams/readable:1701:12)
> ```

Removing or renaming `header=a` in the `content-type` header, or renaming the `content-type` header to any other name, causes the issue to go away.

## 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.