Fix bug where the prerendered doc would contain nested html tags

The 'body' parameter returned from request() is the full document, so pass that directly to `jsdom.jsdom()`. When initializing jsdom with an empty string as the doc as we were doing, jsdom creates `<html>...</html>`, and we were getting nested `<html>` and `<body>` elements.

A before and after example of what I'm referring to:

345c23dc8019afe94ad20352f861dd6d5525f597
![screen shot 2014-05-20 at 7 55 21 pm](https://cloud.githubusercontent.com/assets/59292/3034736/4313d728-e07a-11e3-9868-74b7fef1397f.png)

f57d17539789b2e0acbd91365e69b93b5e59e624
![screen shot 2014-05-20 at 7 56 26 pm](https://cloud.githubusercontent.com/assets/59292/3034742/63065560-e07a-11e3-9392-78e248c55324.png)


I also have a test in here that I've xit'ed out because I couldn't figure out yet how to look at the entire document in karma-jasmine or whatever this is.