fix comment about which mail headers are excluded

actionmailer/lib/action_mailer/base.rb

@@ -820,7 +820,7 @@ def mail(headers = {}, &block)
       # Set configure delivery behavior
       wrap_delivery_behavior!(headers.delete(:delivery_method), headers.delete(:delivery_method_options))
 
-      # Assign all headers except parts_order, content_type and body
+      # Assign all headers except parts_order, content_type, body, template_name, and template_path
       assignable = headers.except(:parts_order, :content_type, :body, :template_name, :template_path)
       assignable.each { |k, v| m[k] = v }