Fix searching elements by class name from a tree including a text node by implementing the missing class_list method

Author: kaapa

lib/arbre/html/text_node.rb

@@ -22,6 +22,10 @@ def build(string)
         @content = string
       end
 
+      def class_list
+        []
+      end
+
       def tag_name
         nil
       end