etcdhttp/etcdserver: support HEAD on /v2/keys/ namespace

fixes #1590 

```
$ curl http://127.0.0.1:4001/v2/keys/foo -I
HTTP/1.1 200 OK
Content-Type: application/json
X-Etcd-Cluster-Id: e9c7614f68f35fb2
X-Etcd-Index: 34
X-Raft-Index: 537
X-Raft-Term: 9
Date: Tue, 04 Nov 2014 06:01:00 GMT
```
http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.13