Fix the leak mentioned in 504a3b5e by another way

The previous fix introduced another leak; as it made
Banzai::Filter::SanitizationFiler#customized? always return false, so we
were always appending two elements to
HTML::Pipeline::SanitizationFilter::WHITELIST[:elements]. This growth in
the elements array would slow the sanitization process over time.

CHANGELOG

@@ -4,6 +4,7 @@ v 8.13.0 (unreleased)
   - Speed-up group milestones show page
 
 v 8.12.1 (unreleased)
+  - Fix a memory leak in HTML::Pipeline::SanitizationFilter::WHITELIST
 
 v 8.12.0
   - Update the rouge gem to 2.0.6, which adds highlighting support for JSX, Prometheus, and others. !6251

lib/banzai/filter/sanitization_filter.rb

@@ -43,17 +43,18 @@ module Banzai
         whitelist[:protocols].delete('a')
 
         # ...but then remove links with unsafe protocols
-        whitelist[:transformers].push(remove_unsafe_links)
+        whitelist[:transformers].push(self.class.remove_unsafe_links)
 
         # Remove `rel` attribute from `a` elements
-        whitelist[:transformers].push(remove_rel)
+        whitelist[:transformers].push(self.class.remove_rel)
 
         # Remove `class` attribute from non-highlight spans
-        whitelist[:transformers].push(clean_spans)
+        whitelist[:transformers].push(self.class.clean_spans)
 
         whitelist
       end
 
+      class << self
         def remove_unsafe_links
           lambda do |env|
             node = env[:node]
@@ -87,7 +88,7 @@ module Banzai
             return unless node.name == 'span'
             return unless node.has_attribute?('class')
 
-          unless has_ancestor?(node, 'pre')
+            unless node.ancestors.any? { |n| n.name.casecmp('pre').zero? }
               node.remove_attribute('class')
             end
 
@@ -96,4 +97,5 @@ module Banzai
         end
       end
     end
+  end
 end