[whatwg/fetch] Cookies not sent across origins with credentials: 'include' and Content-Type: application/json (#997)

When I don't include the `Content-Type` header, cookies are sent:

```js
await fetch('https://localhost:44348/api/test', { credentials: 'include', method: 'POST' })
```

But when I specify the header, the cookies aren't sent:

```js
await fetch('https://localhost:44348/api/test', { credentials: 'include', method: 'POST', headers: { 'Content-Type': 'application/json' } })
```

Why is this?

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