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_.

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

…functions