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.
Merge pull request #111 from magnars/fix-responseText-in-IE-lte-8

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