Improve unhandled request passthrough

This is a spiked implementation that allows this fork (and thereby, Pretender) to be used with [handlebars-require-plugin](https://github.com/SlexAxton/require-handlebars-plugin).

It pulls over the old request's onreadystatechange, including the readyState and responseText from the old request.

I'm not sure what tests may need to be added for this and could use some guidance.