Custom HTTP headers are absent in Fastly Next.js

Custom HTTP headers, set by the headers key in next.config.js or utilized by the NextResponse API in middleware.ts, are absent in Fastly.

Steps to reproduce

Node version: 20.9.0

1. Clone this repo
2. Install dependencies: npm install && npm install @fastly/next-compute-js@alpha
3. Run in dev mode: npm run dev
4. Open Chrome DevTools in Google Chrome and click the Network tab
5. Go to http://localhost:3000
6. Click on the localhost entry in the Network Log table. You will notice that the custom headers, X-Hello-From-Middleware and X-Hello-From-Next-Config, are present in the 'Response Headers' section under the 'Headers' tab.
7. Run in production mode: npm run build && npm run start
8. Custom HTTP header's are still present
9. Run with Fastly: npm run fastly-serve
10. Go to http://localhost:7676 and notice that the custom HTTP headers are absent

Why this is an issue

Without custom headers, one cannot implement HTTP security headers to protect against a range of common web application vulnerabilities.