How to view all response headers in ember with RESTAdapte

My response headers look like this

HTTP/1.1 200 OK
Server: nginx/1.9.7
Content-Type: application/json
Transfer-Encoding: chunked
Connection: keep-alive
X-On-Trial: 1
Cache-Control: no-cache
Access-Control-Allow-Origin: http://localhost:4200
Vary: Origin
Date: Sun, 29 May 2016 00:37:31 GMT

But when I do a console.log(headers) in the RESTAdapter handleResponse function, all that is included is

EmptyObject {Content-Type: "application/json", Cache-Control: "no-cache"}

How can I access the X-On-Trail header or any other custom headers I may need?

I'm not sure if this matters but I am using ember-simple-auth. Does that strip out headers?