fixed host / port detection in request url

This is a follow up fix for both https://github.com/jurmous/etcd4j/pull/7 and https://github.com/jurmous/etcd4j/pull/14: when the value(!) for an etcd request contains an url, requestUrl.contains("://") is always true, even if the request url doesn't contain host and port. A local redirect without host and port happens when the key contains a leading slash.