Edgemicro intro plugin appending to response body instead of replacing

[#1]

I'm working through the first custom plugin tutorial for edgemicro found here: http://docs.apigee.com/microgateway/latest/develop-plugins. I've run into some peculiar behavior though.

Here is my plugin code:

'use strict';
  
var debug = require('debug')('plugin:response-override');


module.exports.init = function(config, logger, stats) {


  return {
   
    ondata_response: function(req, res, data, next) {
      debug('***** plugin ondata_response');
      next(null, null);
    },
    
    onend_response: function(req, res, data, next) {
      debug('***** plugin onend_response');
      next(null, "Hello, World!\n\n");
    }
  };
}

Instead of replacing the response body with custom text. My plugin simply appends the text to the current response body.

HTTP/1.1 200 OK
content-type: application/json; charset=utf-8
date: Mon, 10 Oct 2016 14:31:11 GMT
etag: W/"1cd-igZDs61JMAiJo6ZDx/UFDQ"
x-powered-by: Apigee
x-response-time: 635
Connection: keep-alive
Transfer-Encoding: chunked

{"headers":{"host":"mocktarget.apigee.net","accept":"*/*","user-agent":"curl/7.43.0","via":"undefined1.1 localhost","x-authorization-claims":"eyJzY29wZXMiOltdfQ==","x-forwarded-host":"undefinedlocalhost:8000","x-request-id":"d68c00d0-8ef5-11e6-8c68-5758f301a7dc.30bc13b0-8ef6-11e6-8c68-5758f301a7dc","x-forwarded-for":"undefined::1, 68.49.164.231","x-forwarded-port":"80","x-forwarded-proto":"http","connection":"keep-alive"},"method":"GET","url":"/","body":""}Hello, World!

Any ideas on what my issue is?