properly unlock locked discussions with a lock_at time.

fixes CNVS-6762

test plan:
  * create a discussion with a lock_at in the past;
  * navigate to the discussion index page and drag the
    discussion from "closed for comments" to "discussions;"
  * refresh the page and verify that the topic remains
    unlocked.

Change-Id: I35c819895d41e6f0f70a76c0e8805b963ff41a9e
Reviewed-on: https://gerrit.instructure.com/22111
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Zach Pendleton <zachp@instructure.com>
Reviewed-by: Mark Ericksen <marke@instructure.com>
Product-Review: Mark Ericksen <marke@instructure.com>

app/controllers/discussion_topics_controller.rb

@@ -487,8 +487,9 @@ class DiscussionTopicsController < ApplicationController
       # Handle locking/unlocking (overrides workflow state if provided). It appears that the locked param as a hash
       # is from old code and is not being used. Verification requested.
       elsif params.has_key?(:locked) && !params[:locked].is_a?(Hash)
-        @topic.delayed_post_at = ''
-        @topic.lock_at = ''
+        discussion_topic_hash[:delayed_post_at] = nil
+        discussion_topic_hash[:lock_at]         = nil
+
         if value_to_boolean(params[:locked])
           @topic.lock
         else

spec/controllers/discussion_topics_controller_spec.rb

@@ -307,4 +307,20 @@ describe DiscussionTopicsController do
     end
 
   end
+
+  describe "PUT: update" do
+    before(:each) do
+      course_with_teacher_logged_in(active_all: true)
+      @topic = DiscussionTopic.create!(context: @course, title: 'Test Topic',
+        lock_at: '2013-01-01T00:00:00UTC', locked: true)
+    end
+
+    it "should unlock discussions with a lock_at attribute" do
+      put('update', course_id: @course.id, topic_id: @topic.id,
+          title: 'Updated Topic', format: 'json', lock_at: @topic.lock_at,
+          locked: false)
+
+      @topic.reload.should_not be_locked
+    end
+  end
 end