cheese IS vegetarian, after all.  Hallelujah!!
refactor Pizza class tests for clarity
modify Pizza.vegetarian? tests to use more descriptive test object names
implement Pizza#vegetarian?; fix test calling code bug -- test objects should wrap toppings in an array
failing test added for Pizza.add_topping method
implement Pizza.add_topping method
added failing tests for nil delivery time on Pizza objects and for Pizza#deliver! method
attr_reader on Pizza class auto-sets delivery_time to nil; passes test
added test for Pizza#late? method
implemented .late? method on Pizza class
flipped test logic for Pizza#late? method