Fix initial loading bug.

Change the `MathJax.Hub.Queue(initializeRequest)` to a simpler function call
(`initializeRequest.call(this)`). This was failing to give a proper context
to initializeRequest, and `this.value` was turning up as `undefined`.

Also add a fallback if we need to display some code before MathJax finishes
its original typesetting.

I was stubbing out `Queue` in my specs, so the tests had to be changed around
a little.