message: Fix generation of empty responses on tests
  The right syntax to generate a Response without a body is `new Response(new Blob(), initOpts)`.
  Before this changes, all those invocations were generating 200 OK responses:
  ```
  jsonapiResponse(404)
  jsonapiResponse({status: 404});
  jsonapiResponse({status: 404}, null);
  jsonapiResponse({status: 404, headers: { 'Content-Type': 'application/vnd.api+json' }, null);
  ```
diff: @@ -16,7 +16,7 @@ export function jsonapiResponse(_options: any, body?: any, timeout?: number): Pr
  options.headers['Content-Type'] = options.headers['Content-Type'] || 'application/vnd.api+json';
  response = new Orbit.globals.Response(JSON.stringify(body), options);
  } else {
  - response = new Orbit.globals.Response(options);
  + response = new Orbit.globals.Response(null, options);
  }
  // console.log('jsonapiResponse', body, options, response.headers.get('Content-Type'));
  