Revert libxml2 behavior in Nokogiri gem that could cause XSS

MRI Behavior in libxml2 has been reverted which caused CVE-2018-8048 loofah gem, CVE-2018-3740 sanitize gem, and CVE-2018-3741 rails-html-sanitizer gem. The commit in question is here: https://github.com/GNOME/libxml2/commit/960f0e2 and more information is available about this commit and its impa...