SPIGOT-3336: HideFlags will now serialize as a List rather than a Set.

HideFlags will be serialized as a list and deserialized as an Iterable.
This will allow maximum flexibility on both the formats it can be
serialized into and formats it can be deserialized from.

src/main/java/org/bukkit/craftbukkit/inventory/CraftMetaItem.java

@@ -384,7 +384,7 @@ class CraftMetaItem implements ItemMeta, Repairable {
             setRepairCost(repairCost);
         }
 
-        Set hideFlags = SerializableMeta.getObject(Set.class, map, HIDEFLAGS.BUKKIT, true);
+        Iterable<?> hideFlags = SerializableMeta.getObject(Iterable.class, map, HIDEFLAGS.BUKKIT, true);
         if (hideFlags != null) {
             for (Object hideFlagObject : hideFlags) {
                 String hideFlagString = (String) hideFlagObject;
@@ -777,7 +777,7 @@ class CraftMetaItem implements ItemMeta, Repairable {
             builder.put(REPAIR.BUKKIT, repairCost);
         }
 
-        Set<String> hideFlags = new HashSet<String>();
+        List<String> hideFlags = new ArrayList<String>();
         for (ItemFlag hideFlagEnum : getItemFlags()) {
             hideFlags.add(hideFlagEnum.name());
         }