NSMutableDictionary를 json으로 변환

NSMutableDictionary *requestDict = [[NSMutableDictionary alloc]init];
[requestDict setValue:uuid forKey:@"key"];

NSData *requestData = [NSJSONSerialization dataWithJSONObject:requestDict options:0 error:nil];

NSString *requestJson = [[NSString alloc] initWithData:requestData
                                          encoding:NSUTF8StringEncoding];