Polish RedisZSet.

Improve Javadoc. Add static factory methods for easier RedisZSet construction.

See #2007
Original Pull Request: #2088

src/test/java/org/springframework/data/redis/support/collections/RedisZSetIntegrationTests.java

@@ -36,7 +36,7 @@ public class RedisZSetIntegrationTests extends AbstractRedisZSetTestIntegration<
 	}
 
 	RedisStore copyStore(RedisStore store) {
-		return new DefaultRedisZSet(store.getKey().toString(), store.getOperations());
+		return RedisZSet.create(store.getKey(), store.getOperations());
 	}
 
 	AbstractRedisCollection<Object> createCollection() {