res.send(ArrayBuffer) silently sends {} as JSON

## Bug

`res.send()` handles `Buffer` and TypedArray (`ArrayBuffer.isView`) as binary data, but a raw `ArrayBuffer` — the standard JavaScript binary data type — falls through to `this.json(chunk)`, producing {}` with `Content-Type: application/json`.

```js
res.send(new ArrayBuffer(10))
// Sends: {} (Content-Type: application/json)
// Expected: 10 zero bytes (Content-Type: application/octet-stream)
```

## Cause

PR #6285 (merged as 55869f49) fixed `Uint8Array`/`DataView` support by adding an `ArrayBuffer.isView()` check. However, `ArrayBuffer.isView(new ArrayBuffer(...))` returns `false`, so raw `ArrayBuffer` was missed.

## Proposed fix

Add an `instanceof ArrayBuffer` check in `lib/response.js` that converts to `Buffer` and sets the binary content type, consistent with the existing `ArrayBuffer.isView` branch.

## Real-world impact

`ArrayBuffer` is the standard binary type used by fetch, Web Crypto, WebSocket, FileReader, and similar APIs. Passing it directly to `res.send()` is a common scenario that currently fails silently.

Start in lib/response.js and compare raw ArrayBuffer handling with the existing Buffer and ArrayBuffer.isView paths. Done means res.send(new ArrayBuffer(10)) sends 10 zero bytes with application/octet-stream instead of JSON.

Written by the indexing model from the issue text.