Fix id-dependant promotion code spec failure

This spec wasn't testing what it was intending to. The "code" it was
submitting wasn't a string, but a Spree::PromotionCode record. Our route
helper helpfully called to_param to submit the PromotionCode's id.

This spec only passed when the promotion_code id matched the promotion
id.

api/spec/requests/spree/api/promotions_controller_spec.rb

@@ -38,7 +38,7 @@ module Spree
         end
 
         context 'when finding by code' do
-          let(:id) { promotion.codes.first }
+          let(:id) { promotion.codes.first.value }
 
           it_behaves_like "a JSON response"
         end