Retrieve fake products from the API (bug 1052064)

I'm not crazy about things that look like:

```javascript
var foo = doSomeWork();
if (bar) {
  foo = overwriteThatWork();
}
```

I'd suggest moving the initial assignment to `url` into an `else` but since string creation isn't hard work that's just style.

```javascript
var url;
if (settings.fakeProducts) {
  settings.log.warn('about to fetch fake products');
  url = '/payments/stub-in-app-products/';
} else {
  url = '/payments/' + origin + '/in-app/';
}
```