xhr request race condition readyState already 4 when it should be 1,2,3 ..

[eskimor]

When registering onreadystatechange for an XHR, the handler will be called three times with xmlHttpRequestGetReadyState retrieving already 4 every time, instead of just the last time.

This PR suggests this might be a regression in either ghcjs-dom or jsaddle (the PR refers to ghcjs-dom but also refers to versions that only exist for jsaddle).

We were able to fix this issue for reflex-dom by means of this, but the issue obviously lies deeper.

[ryantrinkle]

It's because of this: ghcjs/jsaddle-dom#14

I'm not sure why it's marked async or what the correct solution is.