Setting the response seems to work fine using your example:
$ curl localhost:8000 -v
* Rebuilt URL to: localhost:8000/
* Trying 127.0.0.1...
* Connected to localhost (127.0.0.1) port 8000 (#0)
> GET / HTTP/1.1
> Host: localhost:8000
> User-Agent: curl/7.47.0
> Accept: */*
>
< HTTP/1.1 200 OK
< Host: localhost:8000
< Connection: close
< X-Powered-By: PHP/7.0.4-7ubuntu2
< Content-type: application/json; charset=utf-8
< Content-Length: 14
<
* Closing connection 0
["YOUR_ARRAY"]
What headers and response body are you seeing?