Setting cookie with invalid character hangs mocha process

I'm having trouble with mocha and supertest. It seems that, when I provide a cookie with an invalid character, my test doesn't exit. The process keeps running.

## Steps to reproduce

Create a new project with `npm init -y` and install dependencies:
```
npm i supertest mocha express
```

Create an `index.js` file with content:s
```
const request = require('supertest');
const express = require('express');

require('mocha');

const app = express();

app.get('/user', function (req, res) {
res.status(200).json({ name: 'john' });
});

describe('app', function() {
it('should exit', function(done) {
request(app)
.get('/user')
.set('Cookie', '‘foo')
.expect('Content-Type', /json/)
.expect('Content-Length', '15')
.expect(200)
.end(function (err, res) {
done(err);
});
});
});
```

Run the test with:
```
./node_modules/.bin/mocha index.js
```

## Expected

An error about an invalid character (`‘` in the cookie) and the process to exit.

## Actual

The error appears but the process doesn't exit.

When I look at the active handles, it seems the server is still running (though I'm no Node internals expert).

## What I've tried

- Return the promise instead of calling `done`
- Adding a `catch` to the end of the chain and calling `done` there
- Wrapping the whole thing in a try-catch