[whatwg/fetch] Enusre that a Request object can be used as RequestInit (Issue #1486)

When constructing a `Request` object, this works in every browser:
```js
const base = new Request('https://example.com', { headers: { 'accept': 'text/html' }});
const request = new Request('https://new.example.com', base);
console.log(request.headers.get('accept')); // text/html
console.log(request.url); // https://new.example.com

```
| Browser      | Works |
| ----------  | ------- |
| Firefox       | ✅         |
| Chrome     | ✅         |
| Safari        | ✅         |
| Deno         | ✅         |

I assumed this was the _prefered_ way to make modifications to a URL of a request object, given that this does **not** work in any browser given that it drops the `headers` from the `base` completely.
```js
const base = new Request('https://example.com', { headers: { 'accept': 'text/html' }});
const request = new Request('https://new.example.com', { ...base });
console.log(request.headers.get('accept')); // null
console.log(request.url); // https://new.example.com

```
| Browser      | Works |
| ----------  | ------- |
| Firefox       | ❌         |
| Chrome     | ❌         |
| Safari        | ❌         |
| Deno         | ❌         |

This seems straightforward enough, if you need to change the URL of a request object, pass the existing request object as the second parameter.

Things get _really_ weird though when their is a body on the request.

```js
const base = new Request('https://example.com', { method: 'POST', body: 'hello!', headers: { 'accept': 'text/html' }});
const request = new Request('https://new.example.com', base);
console.log(request.headers.get('accept')); // text/html
console.log(request.url); // https://new.example.com

console.log(await request.text()); // hello!
```
| Browser      | Works | Result |
| ----------  | ------- | ------- |
| Firefox       | ❌         | `body` is dropped
| Chrome     | ❌         | `Uncaught TypeError: Failed to construct 'Request': The `duplex` member must be specified for a request with a streaming body` |
| Safari        | ✅         | 
| Deno         | ✅         |

Could the spec clarify that Request should work as a RequestInit?

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