[whatwg/fetch] Optional disable setting headers keys in lowercase (#304)

Hi, I'm trying to use `fetch` in Chrome but I think it's not working for me for one reason. I'll explain my use case and you can tell me if I can do something to use `fetch`.

I'm developing a Chrome extension (a content script) that reads from `localStorage` an auth `token`. This way I can make request agains a server. I'm not managing that server and I'm not the owner. I can't change anything on the server. The thing is that I'm trying this:

```
fetch(url, {
  method: "GET",
  headers: {
    "Token": "auth-token-content-goes-hear"
  },
})
```

But when I see the request on Chrome Dev Tools I see `token` instead of `Token` and the server is responding with `401`. My first implementation is doing the same with jQuery and the server is returning results.

I understand the the standard is send headers in lowercase. But maybe could be posible set a flag to skip it. Other wise in situations like mine (user can not change server) is not posible to use `fetch`. 

BTW great work! I appreciate all the work you are doing here 💪 

Thanks!

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