Bring back memory optimization of caching lowercased node name

In 4.6.2, rgrove#175 significantly cut down on memory usage by caching the lowercase
version of the node name. However, as reported in rgrove#177, this broke certain
transformers that modified the node name since the name was cached. We can
bring back this optimization by updating the node name only if it has been
changed.

Closes rgrove#184

lib/sanitize.rb

@@ -218,6 +218,8 @@ def to_html(node)
   end
 
   def transform_node!(node, node_whitelist)
+    node_name = node.name.downcase
+
     @transformers.each do |transformer|
       # Since transform_node! may be called in a tight loop to process thousands
       # of items, we can optimize both memory and CPU performance by:
@@ -229,14 +231,18 @@ def transform_node!(node, node_whitelist)
       config = @transformer_config
       config[:is_whitelisted] = node_whitelist.include?(node)
       config[:node] = node
-      config[:node_name] = node.name.downcase
+      config[:node_name] = node_name
       config[:node_whitelist] = node_whitelist
 
       result = transformer.call(config)
 
       if result.is_a?(Hash) && result[:node_whitelist].respond_to?(:each)
         node_whitelist.merge(result[:node_whitelist])
       end
+
+      # It's possible that a transformer modifies the node name, so only
+      # copy the string again if it has changed.
+      node_name = node.name.downcase if node.name != node_name
     end
 
     node