Submission api no longer causes 500 error

A regression happened that broke SIS grade pass back in the following
gerrit:

https://gerrit.instructure.com/#/c/62210/

fixes CNVS-23599

Test plan:
* Create a course
* Create an assignment
* Create a submission for that assignment
* Make an api call with:
   /api/v1/courses/<courseid>/assignments/<assignmentid>/submissions?include[]=user
* Ensure an internal server error does not occur and that the submission
  json includes a user for each submission.

Change-Id: I1c5bf4d88fe2e8ff8f9b73878e0860a06a566b7c
Reviewed-on: https://gerrit.instructure.com/63970
Tested-by: Jenkins
Reviewed-by: Derek Bender <djbender@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Steven Burnett <sburnett@instructure.com>

Author: Andrew Butterfield

lib/api/v1/submission.rb

@@ -65,7 +65,7 @@ def submission_json(submission, assignment, current_user, session, context = nil
     end
 
     if includes.include?("user")
-      hash['user'] = user_json(submission.user, user, session, ['avatar_url'], submission.context, nil)
+      hash['user'] = user_json(submission.user, current_user, session, ['avatar_url'], submission.context, nil)
     end
 
     if assignment && includes.include?('user_summary')

spec/apis/v1/submissions_api_spec.rb

@@ -126,6 +126,18 @@ def submit_homework(assignment, student, opts = {:body => "test!"})
       expect(json.size).to eq 1
     end
 
+    it "should include user" do
+      json = api_call(:get,
+        "/api/v1/sections/#{@section.id}/assignments/#{@a1.id}/submissions.json",
+        { :controller => 'submissions_api', :action => 'index',
+          :format => 'json', :section_id => @section.id.to_s,
+          :assignment_id => @a1.id.to_s },
+        { :include => %w(user) })
+      expect(json.size).to eq 1
+      expect(json[0]['user']).not_to be_nil
+      expect(json[0]['user']['id']).to eq(@student1.id)
+    end
+
     it "should return assignment_visible" do
       @course.enable_feature!(:differentiated_assignments)
       json = api_call(:get,