Return inGround when checking Arrow's OnGround state. Fixes BUKKIT-4439

By: EvilSeph <evilseph@gmail.com>

paper-server/src/main/java/org/bukkit/craftbukkit/entity/CraftEntity.java

@@ -184,6 +184,9 @@ public abstract class CraftEntity implements org.bukkit.entity.Entity {
     }
 
     public boolean isOnGround() {
+        if (entity instanceof EntityArrow) {
+            return ((EntityArrow) entity).isInGround();
+        }
         return entity.onGround;
     }