gistfile1.txt

  NSURL *url = [NSURL URLWithString:@"http://cache-tester.herokuapp.com/contacts.json"];

  
  NSMutableURLRequest *req = [NSMutableURLRequest requestWithURL:url];

  [req setValue:@"\"3aa02883a2552b735e60c74ba4100548\"" forHTTPHeaderField:@"If-None-Match"];
  

  AFJSONRequestOperation *op = [AFJSONRequestOperation JSONRequestOperationWithRequest:req
                                                       success:^(NSURLRequest *request, NSHTTPURLResponse *response, id JSON) {
                                                         NSLog(@"%s succes: %d",__FUNCTION__, response.statusCode );
                                                         NSLog(@"%s succes %@", __FUNCTION__, [response.allHeaderFields objectForKey:@"ETag"]);
                                                         
                                                         
                                                       } failure:^(NSURLRequest *request,
                                                                   NSHTTPURLResponse *response,
                                                                   NSError *error, id JSON) {
                                                         NSLog(@"%s failed: %d %@",__FUNCTION__, response.statusCode, error );
                                                       }];
  [op start];