Fix performance regression

actionpack/lib/action_view/template.rb

@@ -204,7 +204,7 @@ def load!
 
     private
       def valid_extension?(extension)
-        Template.template_handler_extensions.include?(extension)
+        !Template.registered_template_handler(extension).nil?
       end
 
       def find_full_path(path, load_paths)

actionpack/lib/action_view/template_handlers.rb

@@ -32,13 +32,17 @@ def template_handler_extensions
       @@template_handlers.keys.map(&:to_s).sort
     end
 
+    def registered_template_handler(extension)
+      extension && @@template_handlers[extension.to_sym]
+    end
+
     def register_default_template_handler(extension, klass)
       register_template_handler(extension, klass)
       @@default_template_handlers = klass
     end
 
     def handler_class_for_extension(extension)
-      (extension && @@template_handlers[extension.to_sym]) || @@default_template_handlers
+      registered_template_handler(extension) || @@default_template_handlers
     end
   end
 end