Fake XHR: set status before readyState HEADERS_RECEIVED

Just a small fix for Sinon's fake XHR. This PR ensures that both status and headers are available when transitioning `readyState` to `HEADERS_RECEIVED` as described in the [W3C working draft of XHR][w3c] and in the [XHR reference on MDN][mdn].

[mdn]: https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest#Properties
[w3c]: http://www.w3.org/TR/XMLHttpRequest/#response