res.respond: allow for thrid parameter (message), refactor shared ops responses and adapt tests

test/api/v3/integration/user/POST-user_buy_mystery_set.test.js

@@ -31,11 +31,7 @@ describe('POST /user/buy-mystery-set/:key', () => {
 
     expect(res.data).to.eql({
       items: JSON.parse(JSON.stringify(user.items)), // otherwise dates can't be compared
-      purchased: {
-        plan: {
-          consecutive: user.purchased.plan.consecutive,
-        },
-      },
+      purchasedPlanConsecutive: user.purchased.plan.consecutive,
     });
     expect(res.message).to.equal(t('hourglassPurchaseSet'));
   });