X-Powered-By header is not removed for favicon images

```
HTTP/1.1 200 OK
Strict-Transport-Security: max-age=15552000; includeSubDomains
X-Download-Options: noopen
Accept-Ranges: bytes
Cache-Control: public, max-age=0
Last-Modified: Thu, 07 Sep 2017 21:10:58 GMT
Date: Thu, 07 Sep 2017 22:20:33 GMT
X-Powered-By: Express
ETag: W/"3e4-15e5e2e7ad0"
Content-Type: image/png
Content-Length: 996
```