Support to see if a method has been memoized for a parameter + method combination

[dahnny012]

Ex.

class Book
class << self
extend memoist
def find(id)
if memoized value is nil
return operation
else
return memozied value
end
end
memoize :find
end
end

Book.find(3) # nil
Book.create!(id: 3)
Book.find(3) # should not hit the memoized value