ActiveRecord::Observer simplification causes callbacks to be fired multiple times

Commit 61f2d52d8 from #6065 ActiveRecord::Observer is not aware of method added by including modules causes the same callback from an observer to be fired multiple times.

For example,

class Developer < ActiveRecord::Base; end

class SpecialDeveloper < Developer; end

class DeveloperObserver < ActiveRecord::Observer
  def before_save(developer)
    puts "i'm called"
  end
end

Calling SpecialDeveloper.create(:name => "foo") will cause "i'm called" to be printed twice.

The problem is the simplication in AR::Observer created separate anonymous callbacks on each descendent

  SpecialDeveloper.before_save { |record| observer.before_save(record) }
  Developer.before_save { |record| observer.before_save(record) }

causing each to run in turn. Attached is a fix that goes back to defining a method on Developer and SpecialDeveloper and creating the callback of the form

  Developer.before_save :_notify_developer_observer_before_save_callback
  SpecialDeveloper.before_save :_notify_developer_observer_before_save_callback