ensure an assignment group exists when editing assignment

this manifested itself in a bunch of tickets from pcsd

Change-Id: I86a23cde6e1e55a8251124677bb0e66a2bd0cd92
Reviewed-on: https://gerrit.instructure.com/5170
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Hudson <hudson@instructure.com>

Author: ryankshaw

app/controllers/assignments_controller.rb

@@ -60,6 +60,7 @@ def show
       return
     end
     if authorized_action(@assignment, @current_user, :read)
+      @context.require_assignment_group
       @assignment_groups = @context.assignment_groups.active
       if !@assignment.new_record? && !@assignment_groups.map(&:id).include?(@assignment.assignment_group_id)
         @assignment.assignment_group = @assignment_groups.first

spec/controllers/assignments_controller_spec.rb

@@ -106,7 +106,7 @@ def course_assignment
       a = @course.assignments.create(:title => "some assignment")
 
       get 'show', :course_id => @course.id, :id => a.id
-      assigns[:assignment_groups].should_not be_nil
+      assigns[:assignment_groups].should_not be_blank
       assigns[:unlocked].should_not be_nil
     end