fix due date change messages to have correct date

broadcast policy  was being kicked off too early 
which caused the previous due date to be sent on a
due date change notification

test plan
 * create an assignment 
 * change the due date
 * the assignment due date change notification 
   should have the correct due date

Change-Id: Icaa26143e36113e8ad001e7ce950e463d7526787
Reviewed-on: https://gerrit.instructure.com/16170
Reviewed-by: Zach Pendleton <zachp@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Adam Phillipps <adam@instructure.com>

app/models/assignment.rb

@@ -87,7 +87,6 @@ class Assignment < ActiveRecord::Base
   validate :frozen_atts_not_altered, :if => :frozen?, :on => :update
 
   acts_as_list :scope => :assignment_group_id
-  has_a_broadcast_policy
   simply_versioned :keep => 5
   sanitize_field :description, Instructure::SanitizeField::SANITIZE
   copy_authorized_links( :description) { [self.context, nil] }
@@ -135,9 +134,12 @@ class Assignment < ActiveRecord::Base
                 :clear_unannounced_grading_changes_if_just_unpublished,
                 :schedule_do_auto_peer_review_job_if_automatic_peer_review,
                 :delete_empty_abandoned_children,
-                :remove_assignment_updated_flag,
                 :validate_assignment_overrides
 
+  has_a_broadcast_policy
+
+  after_save :remove_assignment_updated_flag # this needs to be after has_a_broadcast_policy for the message to be sent
+
   def validate_assignment_overrides
     if group_category_id_changed? 
       # needs to be .each(&:destroy) instead of .update_all(:workflow_state =>