always send quiz due date changed notifications.

fixes CNVS-5102, CNVS-5255

when a quiz due date has changed, always send notifications
to affected users; don't require the editor to check the
"notify users this quiz has changed" checkbox.

test plan:
  * as a teacher, change the due date of an existing quiz
    and save the change without checking the "notify users
    that this quiz has changed" checkbox;
  * verify that affected students with ASAP notifications
    receive a message.
  * verify that these notifications also work as expected
    for quizzes with multiple due dates.

Change-Id: Iba4bc32c2eb7629fd475201938ba7adb345c0e6a
Reviewed-on: https://gerrit.instructure.com/19609
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Zach Pendleton <zachp@instructure.com>
Product-Review: Zach Pendleton <zachp@instructure.com>
QA-Review: Zach Pendleton <zachp@instructure.com>

app/controllers/quizzes_controller.rb

@@ -484,7 +484,7 @@ class QuizzesController < ApplicationController
 
           # quiz.rb restricts all assignment broadcasts if notify_of_update is
           # false, so we do the same here
-          if @quiz.assignment.present? && notify_of_update
+          if @quiz.assignment.present? && (notify_of_update || old_assignment.due_at != @quiz.assignment.due_at)
             @quiz.assignment.do_notifications!(old_assignment, notify_of_update)
           end
           @quiz.reload

spec/controllers/quizzes_controller_spec.rb

@@ -826,7 +826,7 @@ describe QuizzesController do
         @student.messages.detect{|m| m.notification_id == @notification.id}.should_not be_nil
       end
 
-      it "should not send due date changed if notify_of_update is not set" do
+      it "should send due date changed if notify_of_update is not set" do
         course_due_date = 2.days.from_now
         section_due_date = 3.days.from_now
         post 'update', :course_id => @course.id,
@@ -841,7 +841,7 @@ describe QuizzesController do
             }]
           }
 
-        @student.messages.detect{|m| m.notification_id == @notification.id}.should be_nil
+        @student.messages.detect{ |m| m.notification_id == @notification.id }.should_not be_nil
       end
     end
   end