SPIGOT-3605: Spawn eggs not saving internal data

By: md_5 <git@md-5.net>

paper-server/src/main/java/org/bukkit/craftbukkit/inventory/CraftMetaSpawnEgg.java

@@ -6,6 +6,7 @@ import java.util.Map;
 import net.minecraft.server.DataConverterTypes;
 import net.minecraft.server.MinecraftKey;
 import net.minecraft.server.MinecraftServer;
+import net.minecraft.server.NBTBase;
 import net.minecraft.server.NBTTagCompound;
 import org.bukkit.Material;
 import org.bukkit.configuration.serialization.DelegateDeserialization;
@@ -66,6 +67,13 @@ public class CraftMetaSpawnEgg extends CraftMetaItem implements SpawnEggMeta {
         }
     }
 
+    @Override
+    void serializeInternal(Map<String, NBTBase> internalTags) {
+        if (entityTag != null) {
+            internalTags.put(ENTITY_TAG.NBT, entityTag);
+        }
+    }
+
     @Override
     void applyToItem(NBTTagCompound tag) {
         super.applyToItem(tag);