Preheat oven to 350. Prepare a 2-quart casserole dish with cooking spray; set aside. In a large saucepan, cook chicken, onions, and bell peppers until chicken is no longer pink. Stir in tomato sauce, chili powder, salt, and black pepper. Spread chicken mixture into prepared dish. Meanwhile, in a mixing bowl, combine cornbread, milk, spread, and egg whites. Mix until all is moistened. Add cheese into mixture. Spread the cornbread mixture evenly over top chicken mixture in dish. Bake for 20 minutes, or until cornbread top is browned.