[fetch] Reconnect and Retry-After (#116)

It would be very helpful to have a option: `isRetry`

```Javascript
fetch(url, {
    isRetry: true
})
```
If network offline or other network problems, auto reconnect to be successful connect.

If server response code error status, and has HTTP header `Retry-After`,
retry send this request, after time specified value in header.
```
HTTP/1.1 503 Service Unavailable
Retry-After: 3
```

What do you think about it?

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/fetch/issues/116