Generate a warning if `#respond_to?` or `#respond_to_missing?` might be doing something unexpected after upgrading to Ruby 2.0.0.

respond_to_warner.rb

if RUBY_VERSION >= '2.0.0'
  class Object
    alias :_respond_to? :respond_to?
    def respond_to?(symbol, include_all = nil)
      _respond_to?(symbol, include_all || false).tap do |b|
        if b == false && include_all.nil? && _respond_to?(symbol, true)
          warn "WARNING! #{self.class.name} responds to #{symbol}, but it's protected or private (from: #{caller[2].inspect})"
        end
      end
    end

    alias :_respond_to_missing? :respond_to_missing?
    def respond_to_missing?(symbol, include_all = nil)
      _respond_to_missing?(symbol, include_all || false).tap do |b|
        if b == false && include_all.nil? && _respond_to_missing?(symbol, true)
          warn "WARNING! #{self.class.name} responds to missing #{symbol}, but it's protected or private (from: #{caller[2].inspect})"
        end
      end
    end
  end
end