Teach ActiveSupport::Memoizable about private methods

Currently, if you define a private method and later call memoize :your_method, the replacement methods created by Memoizable will be public. E.g.

class Person
  extend ActiveSupport::Memoizable
  private
  def is_bald?; end
  memoize :is_bald?
end

Person.new.is_bald?

will not raise *NoMethodError: private method is_bald?' called*.

The attached patch causes ActiveSupport::Memoizable to prepend all the re-definition of your_method with the private keyword if the original your_method had been a private_instance_method in the first place.

Tests are included.