Hashes hkeys and hvals refactored to return a set instead of a list.

It looks like the order is not guaranted, so a set seems much adapted than a list.

src/main/java/redis/clients/jedis/ShardedJedis.java

@@ -150,12 +150,12 @@ public class ShardedJedis extends BinaryShardedJedis implements JedisCommands {
         return j.hlen(key);
     }
 
-    public List<String> hkeys(String key) {
+    public Set<String> hkeys(String key) {
         Jedis j = getShard(key);
         return j.hkeys(key);
     }
 
-    public List<String> hvals(String key) {
+    public Set<String> hvals(String key) {
         Jedis j = getShard(key);
         return j.hvals(key);
     }