Add sis_source_id to assignment_groups json object

WHY:  For Pinnacle grade pass back, we need to find/create
assignment_groups in Pinnacle using the sis_source_id.  This changeset
exposes that attribute through sis_assignments api.

Test Plan:

In your browser, go to
http://canvas.docker/api/sis/courses/:course_id/assignments

For each assignment you should see a sis_source_id in the
assignment_group

Example:
"assignment_group":
  {"id":4,"name":"quizes","sis_source_id":"0.958053570925386"}

Change-Id: Id62ba0c821a5bbaaeefc4ec1c5fe087397ab1061
Fixes: SIS-2128
Reviewed-on: https://gerrit.instructure.com/82423
Tested-by: Jenkins
Reviewed-by: Jayce Higgins <jhiggins@instructure.com>
Reviewed-by: Josh Simpson <jsimpson@instructure.com>
Reviewed-by: Brad Humphrey <brad@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Oxana

app/models/assignment_group.rb

@@ -20,7 +20,13 @@ class AssignmentGroup < ActiveRecord::Base
 
   include Workflow
 
-  attr_accessible :name, :rules, :assignment_weighting_scheme, :group_weight, :position, :default_assignment_name
+  attr_accessible :assignment_weighting_scheme,
+                  :default_assignment_name,
+                  :group_weight,
+                  :name,
+                  :position,
+                  :rules,
+                  :sis_source_id
 
   attr_readonly :context_id, :context_type
   belongs_to :context, polymorphic: [:course]

lib/api/v1/sis_assignment.rb

@@ -25,7 +25,7 @@ module Api::V1::SisAssignment
   }.freeze
 
   API_SIS_ASSIGNMENT_GROUP_JSON_OPTS = {
-    only: %i(id name).freeze
+    only: %i(id name sis_source_id).freeze
   }.freeze
 
   API_SIS_ASSIGNMENT_COURSE_SECTION_JSON_OPTS = {

spec/lib/api/v1/sis_assignment_spec.rb

@@ -75,6 +75,16 @@ describe Api::V1::SisAssignment do
       course_sections.stubs(:association).returns(OpenStruct.new(:loaded? => true))
     end
 
+    it "assignment groups have name and sis_source_id" do
+      ag_name = 'chumba choo choo'
+      sis_source_id = "my super unique goo-id"
+      assignment_group = AssignmentGroup.new(name: ag_name, sis_source_id: sis_source_id)
+      assignment_1.stubs(:assignment_group).returns(assignment_group)
+      result = subject.sis_assignments_json([assignment_1])
+      expect(result[0]["assignment_group"]["name"]).to eq(ag_name)
+      expect(result[0]["assignment_group"]["sis_source_id"]).to eq(sis_source_id)
+    end
+
     it "returns an empty hash for 0 assignments" do
       assignments = []
       expect(subject.sis_assignments_json(assignments)).to eq([])