don't error if you "close for comments" a delayed post

fixes: #10166

test plan:

create a discussion/announcement and say delay posting
till a future date.
click the "close for comments" button on
the show page
it should not have a page error

Change-Id: I8ddba899b1b832824bdcd4d83b6b31cb407acabf
Reviewed-on: https://gerrit.instructure.com/13220
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jon Jensen <jon@instructure.com>

app/coffeescripts/models/DiscussionTopic.coffee

@@ -11,11 +11,9 @@ define [
 
     defaults:
       discussion_type: 'side_comment'
-      delayed_post_at: null
       podcast_enabled: false
       podcast_has_student_posts: false
       require_initial_post: false
-      assignment: null
       is_announcement: false
 
     dateAttributes: [

app/controllers/discussion_topics_controller.rb

@@ -367,9 +367,11 @@ class DiscussionTopicsController < ApplicationController
       @topic.content_being_saved_by(@current_user)
 
       # handle delayed posting
-      @topic.delayed_post_at = discussion_topic_hash[:delayed_post_at]
-      @topic.workflow_state = 'post_delayed' if @topic.delayed_post_at && @topic.delayed_post_at > Time.now
-      @topic.delayed_post_at = "" unless @topic.post_delayed?
+      if discussion_topic_hash.has_key? :delayed_post_at
+        @topic.delayed_post_at = discussion_topic_hash[:delayed_post_at]
+        @topic.workflow_state = 'post_delayed' if @topic.delayed_post_at && @topic.delayed_post_at > Time.now
+        @topic.delayed_post_at = "" unless @topic.post_delayed?
+      end
 
       #handle locking/unlocking
       if params.has_key? :locked