You will have to set a HTTP response header to indicate what the content type of your response is. You can do this as follows:
1. Add a new Create object activity after creating the HttpResponse object.
2. Set the Entity to System.HttpHeader
3. Set Key to 'Content-Type'.
4. Set Value to 'application/json'.
5. Set the association System.HttpHeaders to $NewHttpResponse, the HttpResponse you've just created in your microflow.
You can now successfully call your REST service and the response will be shown.
If your response contains a different content type than a JSON string, you should adjust the Value of your HTTP Content-Type header accordingly. See https://www.w3.org/Protocols/rfc1341/4_Content-Type.html for more information.
And it gets even better and more straightforward now that I have found the Add->Mappings->Message Definitions in the modeler!