code: protected void addSerializableClass(Class<?> serializable) {
          safeMapper.addMixIn(serializable, UseToStringAsValueMixIn.class);
          fullMapper.addMixIn(serializable, UseToStringAsValueMixIn.class);
      }
docs: This protected method allows a subclass to add to the mappers a class type that can be
  serialized using its toString().
  
  @param serializable The type of class that can be serialized using its toString() method.