Re: [whatwg/fetch] Allow request take AsyncIterable body? (#1291)

We shipped a version of this in the wild as an experiment in node compat, and discovered the following issue that the spec will have to deal with that relates to boxed strings:

```js
new Response(new String("hello world"))
```

Currently, boxed strings are cast to `string`, because they don't match any of the interface types (`ReadableStream`, `Blob`, etc). However if the body type becomes `ReadableStream | Blob | BufferSource | DOMString | async iterable<Uint8Array>` as proposed, boxed string (which implements `[Symbol.iterator]`), would be turned into an `async iterable` and then error on the fact that it yields `string` chunks, not `Uint8Array`. This is a web compat issue.

I have two proposed solutions:

1. Like how we disallow primitive strings casting to `async iterable`, we also disallow the boxed string being cast to an `async iterable`. We'd do this in WebIDL, and it would mean that no webidl `async iterable` argument would allow boxed strings. This would be the simplest.
2. We add `object` to the body type union instead of `async iterable`, and then handle this case in the spec by: checking whether the value is a boxed string, in which case cast to string, otherwise decoding it with the webidl union `string | async iterable<Uint8Array>`

Node does not seem to exhibit this behaviour because Node does not allow `[Symbol.iterable]` bodies, only `[Symbol.asyncIterble]` bodies.

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