Binary endpoints not working with adapter-node

[mukaschultze]

Describe the bug

Endpoints that return an Uint8Array fail with the message

The "chunk" argument must be of type string or an instance of Buffer. Received an instance of Uint8Array

when using adapter-node.

To Reproduce
Use the following endpoint and try to access it on the browser using a build that runs with adapter-node:

import type { RequestHandler } from '@sveltejs/kit';

export const get: RequestHandler = async (request) => {
	const headers = {
		'content-type': 'application/octet-stream'
	};

	return {
		status: 200,
		headers,
		body: new Uint8Array([1,2,3,4,5])
	};
};

Expected behavior
The build should return the correct binary data in the same way as svelte-kit dev and svelte-kit preview.

Stacktraces
No stacktraces, but the issue is caused by

kit/packages/adapter-node/src/server.js

Lines 83 to 89 in 06e1c25

	if (rendered) {
	res.writeHead(rendered.status, rendered.headers);
	res.end(rendered.body);
	} else {
	res.statusCode = 404;
	res.end('Not found');
	}

Information about your SvelteKit Installation:

Diagnostics

System:
OS: Windows 10 10.0.19043
CPU: (12) x64 Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz
Memory: 9.53 GB / 31.80 GB
Binaries:
Node: 14.17.1 - C:\Program Files\nodejs\node.EXE
Yarn: 1.22.5 - C:\Program Files (x86)\Yarn\bin\yarn.CMD
npm: 6.14.13 - C:\Program Files\nodejs\npm.CMD
Browsers:
Chrome: 91.0.4472.124
Edge: Spartan (44.19041.1023.0), Chromium (91.0.864.67)
npmPackages:
@sveltejs/adapter-node: next => 1.0.0-next.32
@sveltejs/kit: next => 1.0.0-next.124
svelte: ^3.34.0 => 3.38.3

Additional context
Issue #1381 mentions this error, but it's closed and its PR is already merged.