add user_id to poll serializer

fixes CNVS-13360
This commit adds the user_id to the poll json response, only for
teachers.

Test plan
- As a teacher, create a poll
- Request that poll via the poll#show endpoint
- It should have a 'user_id' field with the id of the creator
- As a student, request the same poll via the poll#show endpoint (the
  student won't be able to view it without a published poll session)
- The student should not see the 'user_id' field.

Change-Id: Ib0b09d779f3486edaa38a870d20211268be819d3
Reviewed-on: https://gerrit.instructure.com/35693
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Derek DeVries <ddevries@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Josh Simpson <jsimpson@instructure.com>

app/controllers/polling/polls_controller.rb

@@ -45,6 +45,11 @@ module Polling
   #           "example": "2014-01-07T15:16:18Z",
   #           "type": "string",
   #           "format": "date-time"
+  #         },
+  #         "user_id": {
+  #           "description": "The unique identifier for the user that created the poll.",
+  #           "example": 105,
+  #           "type": "integer"
   #         }
   #       }
   #    }

app/serializers/polling/poll_serializer.rb

@@ -2,7 +2,7 @@ module Polling
   class PollSerializer < Canvas::APISerializer
     root :poll
 
-    attributes :id, :question, :description, :total_results, :created_at
+    attributes :id, :question, :description, :total_results, :created_at, :user_id
 
     has_many :poll_choices, embed: :ids
 
@@ -24,7 +24,7 @@ module Polling
     private
 
     def teacher_keys
-      [:total_results]
+      [:total_results, :user_id]
     end
 
     def student_keys

spec/apis/v1/polling/polls_api_spec.rb

@@ -80,6 +80,13 @@ describe Polling::PollsController, type: :request do
         poll_json = json['polls'].first
         poll_json.should have_key("total_results")
       end
+
+      it "returns the id of the user that created the poll" do
+        json = get_show
+        poll_json = json['polls'].first
+        poll_json.should have_key("user_id")
+        poll_json['user_id'].should == @teacher.id.to_s
+      end
     end
 
     context "as a student" do
@@ -94,6 +101,16 @@ describe Polling::PollsController, type: :request do
         poll_json.should_not have_key("total_results")
       end
 
+      it "shouldn't return the id of the user that created the poll" do
+        student_in_course(:active_all => true, :course => @course)
+        session = @poll.poll_sessions.create!(course: @course)
+        session.publish!
+
+        json = get_show
+        poll_json = json['polls'].first
+        poll_json.should_not have_key("user_id")
+      end
+
       it "is unauthorized if there are no published sessions" do
         student_in_course(:active_all => true, :course => @course)
         section = @course.course_sections.create!(name: 'Section 2')