public List<T> unmarshall(JsonUnmarshallerContext context) throws Exception {
        if (context.isInsideResponseHeader()) {
            return unmarshallResponseHeaderToList(context);
        }
        return unmarshallJsonToList(context);
    }
Unmarshalls the response headers or the json doc in the payload to the
list
aws-java-sdk-core/src/main/java/com/amazonaws/transform/ListUnmarshaller.java