Multiple Changes to CustomSet Exercise

1. Add test case to check for duplicates. A Set should check for uniqueness. Many users are implementing internally with an array instead of a hash. Also the example code was fixed.
2. Ruby does not have a concept of list, so replacing to_list with to_a
3. Added additional test case in delete to show that CustomSet.new([1,2,3]) != CustomSet.new([1,2.0,3]). I saw some users confused by this.