models' respond_to? does not honor private

I want to make an attribute truly private, so I tried this:

class Foo < ActiveRecord::Base
  # attribute bar
  private
  def bar
    read_attribute :bar
  end
  def bar=(value)
    write_attribute :bar, value
  end
end

but Foo.new.respond_to?(:bar) returns true, where a normal (non-active-record) instance would return false for private methods.

This patch adds a check in respond_to? to honor private methods that shadow attributes.