Fix persistence on tamed pets. Fixes BUKKIT-3300

With the persistence api introduced, pets did not have their
persistence flag updated to reflect their persistence. This caused
tame ocelots to not persist under specific conditions.

By: feildmaster <admin@feildmaster.com>

paper-server/src/main/java/org/bukkit/craftbukkit/event/CraftEventFactory.java

@@ -236,6 +236,8 @@ public class CraftEventFactory {
         org.bukkit.entity.AnimalTamer bukkitTamer = (tamer != null ? (AnimalTamer) tamer.getBukkitEntity() : null);
         CraftServer craftServer = (CraftServer) bukkitEntity.getServer();
 
+        entity.persistent = true;
+
         EntityTameEvent event = new EntityTameEvent((LivingEntity) bukkitEntity, bukkitTamer);
         craftServer.getPluginManager().callEvent(event);
         return event;