allow Edge Functions to stream a compressed fetch response (#39608)

When a user tried to have the following Edge Function:

```ts
export default () => fetch("https://example.vercel.sh");
```

The Edge Function were failing.

Why is that?

When `fetch` was called, an implicit `Accept-Encoding` header was added
to allow the origin to return a compressed response. Then, the origin
will set the `Content-Encoding` header in the response, to let the
client know that the body needs to be decompressed in order to be read.

That creates an issue though: `response.body` will be a
`ReadableStream<Uint8Array>`, or, a stream that contains binary data
that decodes into _the uncompressed data_ (or, plain text!).

What it means, is that `response.body` is uncompressed data, while
`response.headers.get('content-encoding')` is marking the response body
as compressed payload. This confuses the HTTP clients and makes them fail.

This commit removes the `content-encoding`, `transfer-encoding` and
`content-length` headers from the response, as the Next.js server _always_
streams Edge Function responses.

## Bug

- [ ] Related issues linked using `fixes #number`
- [x] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`

packages/next/server/web/sandbox/sandbox.ts

@@ -6,6 +6,12 @@ import { requestToBodyStream } from '../../body-streams'
 
 export const ErrorSource = Symbol('SandboxError')
 
+const FORBIDDEN_HEADERS = [
+  'content-length',
+  'content-encoding',
+  'transfer-encoding',
+]
+
 type RunnerFn = (params: {
   name: string
   env: string[]
@@ -74,12 +80,16 @@ export const run = withTaggedErrors(async (params) => {
     : undefined
 
   try {
-    return await edgeFunction({
+    const result = await edgeFunction({
       request: {
         ...params.request,
         body: cloned && requestToBodyStream(runtime.context, cloned),
       },
     })
+    for (const headerName of FORBIDDEN_HEADERS) {
+      result.response.headers.delete(headerName)
+    }
+    return result
   } finally {
     await params.request.body?.finalize()
   }

test/e2e/edge-compiler-can-import-blob-assets/app/pages/api/edge.js

@@ -38,20 +38,14 @@ const handlers = new Map([
     'remote-full',
     async () => {
       const url = new URL('https://example.vercel.sh')
-      const response = await fetch(url)
-      const headers = new Headers(response.headers)
-      headers.delete('content-encoding')
-      return new Response(response.body, { headers, status: response.status })
+      return fetch(url)
     },
   ],
   [
     'remote-with-base',
     async () => {
       const url = new URL('/', 'https://example.vercel.sh')
-      const response = await fetch(url)
-      const headers = new Headers(response.headers)
-      headers.delete('content-encoding')
-      return new Response(response.body, { headers, status: response.status })
+      return fetch(url)
     },
   ],
 ])