Unable to set Content-Type Response Header in Cakephp 3.2 - response

Question

I'm having difficulty setting the Content-Type response header using the Cake\Network\Response::header method as outlined here: http://book.cakephp.org/3.0/en/controllers/request-response.html#setting-headers
Essentially, I'm able to set just about any header (even random made up ones), except for the Content-Type header. This leads me to believe there is somewhere else in the core application that is writing the Content-Type header AFTER my controller method has completed.
Here's some code:
Controller:
public function sseTest() {
$this->response->header([
'Cache-Control' => 'no-cache',
'Content-Type' => 'text/event-stream'
]);
$this->viewBuilder()->layout('sse');
$this->set('data', date("r"));
}
Template (sse_test.ctp):
data: <?= $data ?>
Layout (sse.ctp):
<?= $this->fetch('content') ?>
And finally, the AppController:
<?php
/**
* CakePHP(tm) : Rapid Development Framework (http://cakephp.org)
* Copyright (c) Cake Software Foundation, Inc. (http://cakefoundation.org)
*
* Licensed under The MIT License
* For full copyright and license information, please see the LICENSE.txt
* Redistributions of files must retain the above copyright notice.
*
* #copyright Copyright (c) Cake Software Foundation, Inc. (http://cakefoundation.org)
* #link http://cakephp.org CakePHP(tm) Project
* #since 0.2.9
* #license http://www.opensource.org/licenses/mit-license.php MIT License
*/
namespace App\Controller;
use Cake\Controller\Controller;
use Cake\Event\Event;
/**
* Application Controller
*
* Add your application-wide methods in the class below, your controllers
* will inherit them.
*
* #link http://book.cakephp.org/3.0/en/controllers.html#the-app-controller
*/
class AppController extends Controller
{
/**
* Initialization hook method.
*
* Use this method to add common initialization code like loading components.
*
* e.g. `$this->loadComponent('Security');`
*
* #return void
*/
public function initialize()
{
parent::initialize();
$this->loadComponent('RequestHandler');
$this->loadComponent('Flash');
}
/**
* Before render callback.
*
* #param \Cake\Event\Event $event The beforeRender event.
* #return void
*/
public function beforeRender(Event $event)
{
if (!array_key_exists('_serialize', $this->viewVars) &&
in_array($this->response->type(), ['application/json', 'application/xml'])
) {
$this->set('_serialize', true);
}
}
}
As you can see, I've got nothing funky in the AppController, and I've dumbed my controller method down to be very basic and just return a formatted date. When I access this page by going to app.com/controller/sse-test I get a page that is exactly what I would expect:
data: Thu, 30 Jun 2016 01:18:03 +0000
The issue is that when I use Chrome Dev Tools to look at the response headers, this is what I have:
HTTP/1.1 200 OK
Date: Thu, 30 Jun 2016 01:18:03 GMT
Server: Apache/2.4.7 (Ubuntu)
X-Powered-By: PHP/5.5.9-1ubuntu4.14
Expires: Thu, 19 Nov 1981 08:52:00 GMT
Cache-Control: no-cache
Pragma: no-cache
X-DEBUGKIT-ID: 511a91a6-c2f7-47b6-85bb-39644994e0dd
Content-Length: 37
Keep-Alive: timeout=5, max=17
Connection: Keep-Alive
Content-Type: text/html; charset=UTF-8
As you can see, the Cache-Control header is set, but then the Content-Type header isn't what I specified. I'm fairly certain it's being overwritten, but I just don't know where. Do any of you have some suggestions as to where I should look or why this might be happening?
For grins, I made a standalone PHP page like below:
<?php
header('Content-Type: text/event-stream');
header('Cache-Control: no-cache');
$time = date('r');
echo "data: The server time is: {$time}\n\n";
flush();
?>
This page works flawlessly supplying server sent events to a static html page, which is ultimately my goal...but I'd like to use CakePHP framework...
I'm sure I'm missing something simple, please let me know your thoughts. Thank you.
Per the first comment below, I looked a little further up the page at "Dealing with Content Types". I've modified my controller to be:
public function sseTest() {
$this->response->header([
'Cache-Control' => 'no-cache'
]);
//add text/event-stream type
$this->response->type('text/event-stream');
$this->viewBuilder()->layout('sse');
$this->set('data', date("r"));
}
I've even taken a look at the response->_mimeTypes variable in the Response class, my text/event-stream is being added. If I simply print out the results of $this->response->type() after setting the content type, it returns text/event-stream. But when the response is set, the actual header still includes Content-Type: text/html; charset=UTF-8. Any further ideas?