Properly copy collection references in ItemMeta.clone(). Fixes BUKKIT-3913

When cloning an item, all references are copied to the new item. For
collections, this makes internal changes become visible in both the old and
new items.

In CraftMetaItem, clone was not making copies of the appropriate collections
and has been fixed for non-null values.

In CraftMetaEnchantedBook and CraftMetaPotion, clone was using possible empty
collection references and has been changed to explicitly null-check instead.

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

@@ -106,8 +106,8 @@ class CraftMetaEnchantedBook extends CraftMetaItem implements EnchantmentStorage
     public CraftMetaEnchantedBook clone() {
         CraftMetaEnchantedBook meta = (CraftMetaEnchantedBook) super.clone();
 
-        if (hasStoredEnchants()) {
-            meta.enchantments = new HashMap<Enchantment, Integer>(enchantments);
+        if (this.enchantments != null) {
+            meta.enchantments = new HashMap<Enchantment, Integer>(this.enchantments);
         }
 
         return meta;

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

@@ -490,7 +490,14 @@ class CraftMetaItem implements ItemMeta, Repairable {
     @Override
     public CraftMetaItem clone() {
         try {
-            return (CraftMetaItem) super.clone();
+            CraftMetaItem clone = (CraftMetaItem) super.clone();
+            if (this.lore != null) {
+                clone.lore = new ArrayList<String>(this.lore);
+            }
+            if (this.enchantments != null) {
+                clone.enchantments = new HashMap<Enchantment, Integer>(this.enchantments);
+            }
+            return clone;
         } catch (CloneNotSupportedException e) {
             throw new Error(e);
         }

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

@@ -117,8 +117,8 @@ class CraftMetaPotion extends CraftMetaItem implements PotionMeta {
     @Override
     public CraftMetaPotion clone() {
         CraftMetaPotion clone = (CraftMetaPotion) super.clone();
-        if (hasCustomEffects()) {
-            clone.customEffects = new ArrayList<PotionEffect>(customEffects);
+        if (this.customEffects != null) {
+            clone.customEffects = new ArrayList<PotionEffect>(this.customEffects);
         }
         return clone;
     }