@Override
            protected void writeKeyTo(Output output, int fieldNumber, K key,
                    boolean repeated) throws IOException
            {
                kTo(output, fieldNumber, key, 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