SPIGOT-2892: Fix some clone implementations and add unit test

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

@@ -200,4 +200,11 @@ public class CraftMetaBanner extends CraftMetaItem implements BannerMeta {
     boolean applicableTo(Material type) {
         return type == Material.BANNER;
     }
+
+    @Override
+    public CraftMetaBanner clone() {
+        CraftMetaBanner meta = (CraftMetaBanner) super.clone();
+        meta.patterns = new ArrayList<>(patterns);
+        return meta;
+    }
 }

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

@@ -213,6 +213,15 @@ public class CraftMetaBlockState extends CraftMetaItem implements BlockStateMeta
         return false;
     }
 
+    @Override
+    public CraftMetaBlockState clone() {
+        CraftMetaBlockState meta = (CraftMetaBlockState) super.clone();
+        if (blockEntityTag != null) {
+            meta.blockEntityTag = blockEntityTag.g();
+        }
+        return meta;
+    }
+
     @Override
     public boolean hasBlockState() {
         return blockEntityTag != null;

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

@@ -176,6 +176,9 @@ public class CraftMetaSpawnEgg extends CraftMetaItem implements SpawnEggMeta {
         CraftMetaSpawnEgg clone = (CraftMetaSpawnEgg) super.clone();
 
         clone.spawnedType = spawnedType;
+        if (entityTag != null) {
+            clone.entityTag = entityTag.g();
+        }
 
         return clone;
     }

src/test/java/org/bukkit/craftbukkit/inventory/ItemMetaCloneTest.java

@@ -0,0 +1,22 @@
+package org.bukkit.craftbukkit.inventory;
+
+import java.lang.reflect.Method;
+import org.bukkit.Material;
+import org.junit.Test;
+
+import static org.hamcrest.Matchers.*;
+import static org.junit.Assert.*;
+
+public class ItemMetaCloneTest {
+
+    @Test
+    public void testClone() throws Throwable {
+        for (Material material : ItemStackTest.COMPOUND_MATERIALS) {
+            Class<?> clazz = CraftItemFactory.instance().getItemMeta(material).getClass();
+
+            Method clone = clazz.getDeclaredMethod("clone");
+            assertNotNull("Class " + clazz + " does not override clone()", clone);
+            assertThat("Class " + clazz + " clone return type does not match", clone.getReturnType(), is(equalTo(clazz)));
+        }
+    }
+}