fix: preserve status and headers when compressing a Fetch Response

[priitkaard]

Compressing a Fetch Response drops its status code and headers.

Fastify doesn't unwrap a Response before onSend - it does that afterwards in onSendEnd and
only if the payload is still a Response. We swap in the compressed stream before then, so that
never runs and reply.code()/reply.header() are never called.

Practically this means content-type, Cache-Control and Set-Cookie disappear and every status
becomes 200.

app.register(compress, { threshold: 0 })
app.get('/', (_req, reply) =>
  reply.send(new Response('hello', { status: 401, headers: { 'set-cookie': 'a=b' } })))

accept-encoding: gzip      -> 200, no set-cookie
accept-encoding: identity  -> 401, set-cookie: a=b

Fix applies the status and headers when we take the body, same as onSendEnd would have.

This affects every tRPC v11 app using this plugin, since its Fastify adapter always sends a
Response, and those are streams so the threshold never skips them.
Related issue: trpc/trpc#6628.

Checklist

• run npm run test && npm run benchmark --if-present
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[mcollina]

lgtm