HEAD request to play.etcd.io returns 404

[gyuho]

Copied from etcd-io/etcd-play#62

When I attempt a HEAD request on the playground, I get a 404:

$ curl -v --head http://play.etcd.io/                                                                                ~/code/crawford/marker
*   Trying 104.199.121.254...
* TCP_NODELAY set
* Connected to play.etcd.io (104.199.121.254) port 80 (#0)
> HEAD / HTTP/1.1
> Host: play.etcd.io
> User-Agent: curl/7.53.1
> Accept: */*
>
< HTTP/1.1 404 Not Found
HTTP/1.1 404 Not Found
< Server: nginx/1.10.1 (Ubuntu)
Server: nginx/1.10.1 (Ubuntu)
< Date: Tue, 15 Aug 2017 17:33:34 GMT
Date: Tue, 15 Aug 2017 17:33:34 GMT
< Content-Type: text/html; charset=utf-8
Content-Type: text/html; charset=utf-8
< Content-Length: 140
Content-Length: 140
< Connection: keep-alive
Connection: keep-alive
< X-Powered-By: Express
X-Powered-By: Express
< Access-Control-Allow-Origin: *
Access-Control-Allow-Origin: *
< Content-Security-Policy: default-src 'self'
Content-Security-Policy: default-src 'self'
< X-Content-Type-Options: nosniff
X-Content-Type-Options: nosniff
< Vary: Accept-Encoding
Vary: Accept-Encoding

<
* Connection #0 to host play.etcd.io left intact

When I do a GET, I get a 200. The HEAD request should also return a 200.

/cc @crawford