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
```
or 
```
case "GET" && r.Wait:
  ...
case "GET", "HEAD":
   ...
```

*shrug*