Missing X-Raft-Term & X-Raft-Index headers

According to https://github.com/coreos/etcd/blob/master/Documentation/api.md#response-headers, the following request should return `X-Raft-Term` and `X-Raft-Index` headers. It clearly does not:

```
% curl -i -L http://127.0.0.1:4001/v2/keys/message -XPUT -d value="Hello world"
HTTP/1.1 200 OK
Content-Type: application/json
X-Etcd-Index: 2
Date: Wed, 24 Sep 2014 21:00:10 GMT
Transfer-Encoding: chunked

{"action":"set","node":{"key":"/message","value":"Hello world","modifiedIndex":2,"createdIndex":2},"prevNode":{"key":"/message","value":"Hello world","modifiedIndex":1,"createdIndex":1}}
```