@Override
            protected void transferKey(Pipe pipe, Input input, Output output,
                    int number, boolean repeated) throws IOException
            {
                kTransfer(pipe, input, output, number, repeated);
            }
Since we cannot inherit multiple classes, we create this Map schema simply to delegate to the wrapping class'
abstract methods.
protostuff-runtime/src/main/java/io/protostuff/runtime/RuntimeMapField.java