Allow to compare XML-Fragments in Strings

Hi, 

I found an issue when comparing Strings with NodeLists like "<h1>Headline</h1><h1>Headline</h1>"
and "<h1>Headline</h1><h2>Headline2</h2>". Everything after the first tag got ignored.

The new spec below demonstrates the issue. This patch is a simple fix by utilizing Nokogiris Fragment-Parsing.