api: allow only offer as a param to course update

fixes #10063

the coures update api required passing at least one course[*something*]
parameter, which meant if all you wanted to do was offer the course, you had to
pass a dummy parameter.

test plan:
- call the course update api an only pass {:offer => 1}
- it should work

Change-Id: I5b89f1f3f474c3b097083f17403e1da20be4ca56
Reviewed-on: https://gerrit.instructure.com/13057
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brian Palmer <brianp@instructure.com>

app/controllers/courses_controller.rb

@@ -1107,6 +1107,7 @@ class CoursesController < ApplicationController
   def update
     @course = api_find(Course, params[:id])
     if authorized_action(@course, @current_user, :update)
+      params[:course] ||= {}
       root_account_id = params[:course].delete :root_account_id
       if root_account_id && Account.site_admin.grants_right?(@current_user, session, :manage_courses)
         @course.root_account = Account.root_accounts.find(root_account_id)

spec/apis/v1/courses_api_spec.rb

@@ -269,6 +269,14 @@ describe CoursesController, :type => :integration do
         @course.reload
         @course.end_at.should be_nil
       end
+
+      it "should allow updating only the offer parameter" do
+        @course.workflow_state = "claimed"
+        @course.save!
+        api_call(:put, @path, @params, {:offer => 1})
+        @course.reload
+        @course.workflow_state.should == "available"
+      end
     end
 
     context "a teacher" do