"unsetting" a TTL fails

According to https://github.com/coreos/etcd/blob/master/Documentation/api.md#using-key-ttl, you should be able to set a TTL to an empty value to "unset" it. My attempt to do so failed:

```
% curl -L http://127.0.0.1:4001/v2/keys/foo -XPUT -d value=bar -d ttl=
{"errorCode":202,"message":"The given TTL in POST form is not a number","cause":"invalid value for \"ttl\"","index":0}
```
in the implementation for 0.4.6, it means that it expires now.