code: final DynamoDBMapperConfig merge(final DynamoDBMapperConfig overrides) {
          return overrides == null || this == overrides ? this : builder().merge(this).merge(overrides).build();
      }
docs: Merges these configuration values with the specified overrides; may
  simply return this instance if overrides are the same or null.
  @param overrides The overrides to merge.
  @return This if the overrides are same or null, or a new merged config.