使用AFNetworking下载文件时，为期设置请求头，如下：

NSString * urlStr = [NSString stringWithFormat:urlString];
NSMutableURLRequest *request = [[AFHTTPRequestSerializer serializer] requestWithMethod:@"GET" URLString:urlStr parameters:nil error:nil];
    [request setValue:@"xxxxxxxx" forHTTPHeaderField:@"sessionId"];
NSURLSessionDownloadTask *downloadTask = [manager downloadTaskWithRequest:request progress:^(NSProgress * _Nonnull downloadProgress) .......