Feature Request: support body from ReadableStream

[cdaringe]

Problem

fetchToCurl(new Request(myUrl, { method: 'POST', body: someReadableStream }) doesn't work, and yields --data-binary={}

Discussion

It's easy to get into wanting this case when dealing with Request instances from common node tools. For example, I'm using msw, which... by the time a request is exiting the VM and you're in a proxy handler, body is converted to a stream just from node itself.

Here's a dummy example of what we could do:

async function reqWithStreamToBodyString(req) {
  const reader = req.clone().body.getReader();
  let result = '';
  while (true) {
    const { value, done } = await reader.read();
    if (done) break;
    result += new TextDecoder("utf-8").decode(value);
  }
  return JSON.stringify(result);
}

Then, rather than a naive if (typeof body === 'object') { ... }, we could first do a if (typeof req.body?.getReader === 'function) { ... } and do the needful