don't override topic assignment for the edit page

fixes CNVS-9783

test plan
- create a graded discussion with a section having available dates
 and 'Everyone Else' having none
- navigate to the edit page
- ensure that the 'Everyone Else' section still has blank available
 dates

Change-Id: Ib34bb7e5664ac9cb3db100b4701cff7ffcaab97d
Reviewed-on: https://gerrit.instructure.com/27919
Reviewed-by: Zach Pendleton <zachp@instructure.com>
Product-Review: Zach Pendleton <zachp@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>

lib/api/v1/discussion_topics.rb

@@ -68,7 +68,7 @@ module Api::V1::DiscussionTopics
 
     locked_json(json, topic, user, session)
     if include_assignment && topic.assignment
-      json[:assignment] = assignment_json(topic.assignment, user, session, include_discussion_topic: false)
+      json[:assignment] = assignment_json(topic.assignment, user, session, include_discussion_topic: false, override_dates: false)
     end
 
     json