Bugfix: Don't assign responseText in IE<=8

- try to assign the filtered response to resp.responseText
 - if it doesn't work (like in Internet Explorer 8 and below), ignore it

This does make for different behavior across browsers. The alternative is
to remove this feature all together.

With this change, the tests run further in IE8. They used to break already at `Setup: dataFilter`. Like IE9, they now freezes at MODULE: Callbacks.

There is already a pull request with this fix (#108), but that one also has some other features bundled along. I figured it was more likely to get a pure bugfix accepted.