[xcode10] Siri shortcuts - custom responses not appearing

### Steps to Reproduce

1. Download [Apple's SoupChef sample](https://developer.apple.com/documentation/sirikit/accelerating_app_interactions_with_shortcuts?language=objc).
2. Run this sample; create a soup order; assign a Siri shortcut to it; invoke the Siri shortcut; to confirm the order, say "Yes" rather than tapping on the button in the UI.
3. Note how after the soup order completes, Siri indicates (audibly) that it will be ready to pick up in ten minutes.
4. Download the Xamarin [SoupChef port](https://github.com/xamarin/ios-samples/tree/master/ios12/SoupChef)
5. Run the same test on the Xamarin port, but note that Siri does not read the custom response text
6. It appears that the correct completion handler _is_ getting called in `OrderSoupIntentHandler` in the `HandleOrderSoup` method, but it's not having the anticipated effect. To see this, debug the **SoupChefIntents** project and set a breakpoint on the last line of the `HandleOrderSoup` method, where the completion handler is called.
- Is there an issue with the way the completion handler is bound? (See the `OrderSoupIntentBinding` project in the same solution)
- Note that the `OrderSoupIntentResponse.SuccessIntentResponseWithSoup` method doesn't seem to be the problem, as I attempted to just manually create the `OrderSoupIntentResponse` object and pass it to the completion handler. Same result.

### Expected Behavior

Siri should indicate audibly (and in text in the UI) that the order will be ready in 10 minutes, as it does in the Swift-based example.

### Actual Behavior

Siri completes the order, but does not indicate that it will be ready in ten minutes.

### Environment

- Xcode 10 beta 5