Fix for prerendered content sometimes prepended by 'null'

Hey - I added a failing test for this issue I was seeing:
![screen shot 2014-05-19 at 10 11 42 pm](https://cloud.githubusercontent.com/assets/59292/3022218/24dfe852-dfc4-11e3-8e93-f34b21b86be6.png)

And I changed some code in the prerenderer to not prepend `document.doctype` to the prerendered content if it's `null`. Let me know if this doesn't make sense.