Make requests in IE8 via JSONP w/o XMLHttpRequest

This fixes the problem when big corportate clients' browsers have 'Enable native XMLHTTP support' turned off. Why that could possibly be considered more secure, nobody knows.

@howardr, using `in` to test for presence is bad, since if you manually set something to `undefined`, the test passes. This is the case in IE8 when XMLHTTP is turned off. The test now works when the value is `null`/`undefined`, and all tests pass (at least, the ones that passed before).