account for POST bodies when serializing fetches

[Rich-Harris]

fixes #633. Figured the pragmatic choice is to ignore requests with a non-string body (as in, they don't get serialized; those fetches are repeated in the client) since other kinds are unlikely in load (presumably the main situation where this is useful is with GraphQL, which uses a JSON payload) and it would be undesirable to buffer streams, etc.

Handling other kinds of bodies is a TODO in any case:

kit/packages/kit/src/runtime/server/page/load_node.js

Lines 148 to 159 in 7679399

	const rendered = await respond(
	{
	host: request.host,
	method: opts.method || 'GET',
	headers,
	path: resolved,
	// TODO per https://developer.mozilla.org/en-US/docs/Web/API/Request/Request, this can be a
	// Blob, BufferSource, FormData, URLSearchParams, USVString, or ReadableStream object
	// @ts-ignore
	rawBody: opts.body,
	query: new URLSearchParams(search)
	},

In fact, I think for now it might be best to throw an error if non-string bodies are sent in fetch in load. Better than silently failing. Though either way it should affect roughly zero people.

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[benmccann]

Additionally supporting JSON doesn't seem like it should be too hard and might be worth it if it helps add GraphQL support. Though presumably those users could just serialize the JSON to a string themselves before calling fetch

[Rich-Harris]

JSON is a string. You don't post body: {...}, you post body: JSON.stringify({...})