[whatwg/xhr] The upload listener flag is not respected in browsers (#95)

Step 4 of [send()](https://xhr.spec.whatwg.org/#the-send()-method) states:

> 5) If one or more event listeners are registered on the associated XMLHttpRequestUpload object, then set upload listener flag. 

Therefore, if any event listeners are added after `send()`, they should be ignored. I find this counter-intuitive; why prevent later event handlers from receiving events? In any case, the browser behavior (tested in latest Firefox and Chrome) does not match the spec.

Form what I can see, the upload listener flag is probably only used to set the `use-CORS-preflight flag`. I suggest removing mentions of the upload listener flag where it determines whether to fire upload progress events.

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