`getServerByKey` cannot report weight anymore

Due to a change in libmemcached (exposed in
5450f45a3ee9ca520c108a043dd10fbe9b5535ac), `getServerByKey` is not able
any more to report a server weight. It reports 0 instead. This makes the
test on this function fails. We fix this by expecting a weight of 0
instead of a weight of 1.