add failing test for jsdom.jsdom('')

I've added a failing test for some strange behavior I'm seeing:

`jsdom.jsdom('')` doesn't initialize an empty dom as expected, it uses the default string - `<html><head></head><body></body></html>`.

The logic here looks fine: https://github.com/tmpvar/jsdom/blob/master/lib/jsdom.js#L74

I'll update this PR if I'm able to get to the bottom of this.