fix to_yaml when dumping ActiveRecord instances with nil association_proxies

to_yaml.rb

module ActiveRecord
  class Base
    def to_yaml_with_nil_handling(opts = {})
      # detect and remove any belongs_to association proxies
      self.class.reflect_on_all_associations.find_all { |a| a.macro == :belongs_to }.each do |a|
        ivar = "@#{a.name}"
        remove_instance_variable(ivar) if instance_variable_defined?(ivar) && instance_variable_get(ivar).class.nil?
      end
      to_yaml_without_nil_handling(opts)
    end
    alias_method_chain :to_yaml, :nil_handling
  end
end