adding correlation id response header

[frankgreco]

$ curl -i localhost:8080
HTTP/1.1 404 Not Found
Content-Type: application/json
X-Kanali-Correlation-Id: d0fb44cc-86f6-4eb7-8e8a-0ccd9da1c8eb
Date: Wed, 02 May 2018 21:35:41 GMT
Content-Length: 173

{"status":404,"message":"No ApiProxy resource was not found that matches the request.","code":0,"details":"Visit https://kanali.io/docs/v2/errorcodes/#00 for more details."}

[coveralls]

Coverage decreased (-0.09%) to 75.897% when pulling 4669c4e on correlation-resp-header into 69efa18 on master.