修正needle在摘要认证时第二次请求仍返回401错误

不知理论上是否正确，但是解决问题了。原因是：needle在第一次收到需要验证信息时，确实是计算出验证信息并将authorization添加到headers里直接返回给服务器，这样服务器就能返回数据。当再次请求服务器时needle就没有往headers里添加authorization了。
处理方法：
needle.js

//添加全局变量authInfo，用来保存authorization
var authInfo = null;
Needle.prototype.send_request = function(count, method, uri, config, post_data, out, callback) {
   ......
  //在发送请求前判断authInfo是否为null,不为null，则重新计算authorization并添加到headers里
  if(authInfo){
    //digest = function(header, user, pass, method, path)
    var auth_header = auth.digest(authInfo, config.credentials[0], config.credentials[1], request_opts.method, request_opts.path);
    if (auth_header) {
      authInfo = auth_header
      request_opts.headers['authorization'] = auth_header
    }
  }
 
  var request = protocol.request(request_opts, function(resp) {
    if (resp.statusCode == 401 && headers['www-authenticate'] && config.credentials) {
      if (!config.headers['authorization']) { // only if authentication hasn't been sent        
        var auth_header = auth.header(headers['www-authenticate'], config.credentials, request_opts);
        if (auth_header) {
          config.headers['authorization'] = auth_header;
          // 保存验证信息
          authInfo = auth_header;
          return self.send_request(count, method, uri, config, post_data, out, callback);
        }
      }
    }
  }
}

auth.js

digest.generate = function(header, user, pass, method, path) {
   ......
   //取出旧的nc值
   if(typeof challenge.nc === 'string'){
    nc = +challenge.nc
  }
  ......
}