fix: wrap task should always return promise (#48)

lib/load_schedule.js

@@ -31,7 +31,7 @@ function getScheduleLoader(app) {
 
         let task;
         if (is.class(schedule)) {
-          task = (ctx, data) => {
+          task = async (ctx, data) => {
             const s = new schedule(ctx);
             s.subscribe = app.toAsyncFunction(s.subscribe);
             return s.subscribe(data);