unsubscribe method
Implementation
Future<HttpClientResponse> unsubscribe(String app) async {
var uri = Uri.http(baseUrl, '/applications/${app}/subscription');
HttpClientRequest request = await client.deleteUrl(uri);
HttpClientResponse response = await request.close();
print(response);
final String stringData = await response.transform(utf8.decoder).join();
print(response.statusCode);
//print(stringData);
return response;
}