check .draft_state_enabled? on correct context in assignments api

fixes CNVS-9059

test plan:
- disable draft state in the console
- turn on draft state in the account and course settings
- create a new assignment
- go to the course assignments page
- click the cloud next to the assignment to publish
- refresh the page, it should still be published

Change-Id: Iddb633b4785a3343594bd64459aa245e991fb234
Reviewed-on: https://gerrit.instructure.com/25720
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cameron Sutter <csutter@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>

lib/api/v1/assignment.rb

@@ -167,8 +167,7 @@ module Api::V1::Assignment
       hash['module_ids'] = module_ids
     end
 
-    #show published/unpublished if account.settings[:enable_draft]
-    if @domain_root_account.enable_draft?
+    if assignment.context.draft_state_enabled?
       hash['published'] = ! assignment.unpublished?
       hash['unpublishable'] = !assignment.has_student_submissions?
     end
@@ -324,7 +323,7 @@ module Api::V1::Assignment
       update_params["description"] = process_incoming_html_content(update_params["description"])
     end
 
-    if @domain_root_account.enable_draft?
+    if assignment.context.draft_state_enabled?
       if assignment_params.has_key? "published"
         published = value_to_boolean(assignment_params['published'])
         assignment.workflow_state = published ? 'published' : 'unpublished'

lib/api/v1/assignment_group.rb

@@ -42,7 +42,7 @@ module Api::V1::AssignmentGroup
       # fake assignment used for checking if the @current_user can read unpublished assignments
       fake = group.context.assignments.new
       fake.workflow_state = 'unpublished'
-      if @domain_root_account.enable_draft? && !fake.grants_right?(user, session, :read)
+      if group.context.draft_state_enabled? && !fake.grants_right?(user, session, :read)
         # user should not see unpublished assignments
         assignment_scope = assignment_scope.published
       end