http needs Marshal()

example/fruit.proto

@@ -55,7 +55,7 @@ message Basket {                                        // `autogenpb:nomutex`
         repeated Apple              stacks        = 6;
 }
 // "Fruit" must exist. you can put anything in it
-message Fruit {                                         // `autogenpb:http`
+message Fruit {                                         // `autogenpb:http` `autogenpb:marshal`
         string                      brand         = 1;  // `autogenpb:unique` `autogenpb:sort`
         Apple                       apples        = 2;
         repeated Pear               pears         = 3;