Make sure to not call res.handle in case the request was aborted in IE9

This PR solves an issue documented in http://stackoverflow.com/questions/7287706/ie-9-javascript-error-c00c023f

When making an XMLHttpRequest in IE9, it will still go ahead and call the `onreadystatechange` handler with `readyState === 4`. Then when trying to access a property of the XHR, it dies with a wonderfully cryptic `SCRIPT575: Could not complete the operation due to error c00c023f.`