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

src/main/java/net/minecraft/server/EntityArrow.java

@@ -444,4 +444,10 @@ public class EntityArrow extends Entity implements IProjectile {
 
         return (b0 & 1) != 0;
     }
+
+    // CraftBukkit start
+    public boolean isInGround() {
+        return inGround;
+    }
+    // CraftBukkit end
 }

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;
     }