Add a failing test for Topping.initialize.
added initialize method for pizza class
add a failing test for topping.veggie
added keyword arguemet for Topping.new
add a failing test for Pizza.initialize
created initialize method for pizza class
failing tet for pizza.new default value
set default topping to cheese
added failing test for pizza.vegetarian
added vegetarian? method
added failing add_toppings method
created add_topping method'