add redo function on command pattern

command/src/main/java/com/iluwatar/InvisibilitySpell.java

@@ -17,6 +17,13 @@ public class InvisibilitySpell extends Command {
 		}
 	}
 
+	@Override
+	public void redo() {
+		if (target != null) {
+			target.setVisibility(Visibility.INVISIBLE);
+		}
+	}
+
 	@Override
 	public String toString() {
 		return "Invisibility spell";