Header added in before middleware is added twice

[jelhan]

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.

   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:

   $ 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

[roncodes]

+1 running into the same issue currently which is causing mozilla observatory to score our website low due to the duplication of security headers.

+1, I accidentally missed the right repo and created the same issue but in the ember-cli-fastboot repo:
ember-fastboot/ember-cli-fastboot#884