Header added in before middleware is added twice

A header added in a before middleware is added twice to the response.

Steps to reproduce:

1. Create a new Ember application.
2. Install `ember-cli-fastboot` and `fastboot-app-server`.
3. Create a FastBoot App Server, which adds a header in a before middleware and safe it as `server.js`.
```js
const FastBootAppServer = require('fastboot-app-server');
const ExpressHTTPServer = require('fastboot-app-server/src/express-http-server');

const httpServer = new ExpressHTTPServer({
port: 49742,
});
const app = httpServer.app;

app.use(function (req, res, next) {
res.set('Content-Security-Policy', "default-src 'http://examples.com';");
next();
});

let server = new FastBootAppServer({
distPath: 'dist',
httpServer: httpServer,
});

server.start();
```
4. Start the FastBoot App Server with `node server.js`.
5. Do a request against the running instance and expect the response headers:
```sh
$ curl -I http://localhost:49742
HTTP/1.1 200 OK
x-powered-by: Express
x-powered-by: Express
content-security-policy: default-src 'http://examples.com';
content-security-policy: default-src 'http://examples.com';
Content-Type: text/html; charset=utf-8
Content-Length: 3314
ETag: W/"cf2-rOuN5P2xOUzKctuV2vXWj7cma28"
Date: Tue, 03 Nov 2020 08:24:54 GMT
Connection: keep-alive
Keep-Alive: timeout=5
```

I expect `Content-Security-Policy` header to be only be present once. But it's present twice.

Please note that I see the same for `X-Powered-By: Express` header, which is added by Express automatically regardless of my usage of FastBoot App Server.

A repository with the reproduction steps applied can be found here: https://github.com/jelhan/test-fastboot-with-header