Use Nokogiri::HTML5#fragment for normalising html

This will use the Nokogump and the Gumbo HTML5 parser for HTML
normalisation, but as this is already required by Sanitize this
should be ok. Changing `parse => fragment` should have no effect
and will stip out superfluous elements such as html, head and body.

Author: erino

lib/govspeak/html_validator.rb

@@ -18,7 +18,7 @@ def valid?
 
   # Make whitespace in html tags consistent
   def normalise_html(html)
-    Nokogiri::HTML.parse(html).to_s
+    Nokogiri::HTML5.fragment(html).to_s
   end
 
   def govspeak_to_html