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/JedisCommands.java

@@ -58,9 +58,9 @@ public interface JedisCommands {
 
     Integer hlen(String key);
 
-    List<String> hkeys(String key);
+    Set<String> hkeys(String key);
 
-    List<String> hvals(String key);
+    Set<String> hvals(String key);
 
     Map<String, String> hgetAll(String key);