fix: use Loofah.fragment for HTML4 parsing

to maintain backwards compatibility with older version of Loofah

lib/rails/html/sanitizer.rb

@@ -39,7 +39,7 @@ def sanitize(html, options = {})
       module Parser
         module HTML4
           def parse_fragment(html)
-            Loofah.html4_fragment(html)
+            Loofah.fragment(html) # TODO: update to html4_fragment when we require Loofah >= 2.21
           end
         end