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.
Merge pull request #14 from nikolas/fix-nested-html-tags

Fix bug where the prerendered doc would contain nested html tags