body mixin returns Uint8Array instead of ArrayBuffer

[terminal69]

Bug Description

According to the documentation the body mixin arrayBuffer() should return Promise, however it returns Promise.

References:
https://github.com/nodejs/undici/blob/main/docs/api/Dispatcher.md#dispatcherrequestoptions-callback
https://fetch.spec.whatwg.org/#body-mixin

Reproducible By

❯ node
Welcome to Node.js v18.18.0.
Type ".help" for more information.
> const undici = require('undici')
undefined
> const response = await undici.request("https://echo.free.beeceptor.com/")
undefined
> const buffer = await response.body.arrayBuffer();
undefined
> typeof buffer
'object'
> buffer instanceof ArrayBuffer
false
> buffer instanceof Uint8Array
true
> typeof buffer.buffer
'object'
> buffer.buffer instanceof ArrayBuffer
true
>

Expected Behavior

Not sure if this is a bug or expected behaviour however the documentation should at least reflect this

Environment

Fedora 38
linux kernel 6.6.0-0.rc4
nodejs 18.18.0
undici@5.24.0

Additional context

If this is a bug that has been present for a long time then fixing it may need to be classified as a breaking change

[terminal69]

Reproduced on undici@5.26.3

> let promiseBuffer = response.body.arrayBuffer()
undefined
> promiseBuffer
Promise {
  Uint8Array(336) [
    ...
  ],
  [Symbol(async_id_symbol)]: 1095,
  [Symbol(trigger_async_id_symbol)]: 6
}
>