Keep receiving 404 "Not Found" with promises in url handler
My trouble is pretty similiar to #886 and #881, but I'm using async function creator. 

```js
function wrapCall(requestCreator) {
    return async function handler(ctx) {
        try {
            const response = await requestCreator(ctx);
            ctx.status = 200;
            ctx.body = {
                ...response,
                ok: true,
            };

        } catch (error) {
            ctx.throw(error.code, error.message);
        }
    };
}

api.get('/operators/', koaBody, wrapCall((ctx) => {
    return adapter.pull('operators', apiModel.getAviableOperatorsList()); // returns promise
}));
```

I'm setting `status=200` but koa sends `404 Not found`. I'm wrapping function that returns promise so I'm expecting that wrapped function will be correctly works in resulting `handler`. 

What am I doing wrong?
