[jQuery] jQuery.ajax() not calling beforeSend?

[TTop]

I need to set some headers (I wish this were doable by just passing in
a map), but it seems that beforeSend is never being called. Needless
to say that's a bit of a problem for me.

The alert below never occurs (and a breakpoint is never hit if I put
it there), but I can see that the ajax request is being sent.

                j$.ajax({
                        'url': serviceUrl,
                        dataType: "jsonp",
                        data: params,
                        beforeSend: function(req) {
                           alert("inside!");
                        }
                });