Update xhr timeout support bug.

Chrome supports it since M29:
  https://code.google.com/p/chromium/issues/detail?id=231959
Firefox supports it since M12:
  https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest#Browser_Compatibility
Merge pull request #237 from MayhemYDG/patch-1

Update xhr timeout support bug.