app.all('*',function(req,res,next))   -->* 代表所有访问 ，

  res.header('Access-Control-Allow-Origin', '*');  ---->代表同意跨域
  res.header('Access-Control-Allow-Headers', 'Content-Type, Content-Length, Authorization, Accept, X-Requested-With ');  ------>代表支付HTTP头字段
  res.header('Access-Control-Allow-Methods', 'PUT, POST, GET, DELETE, OPTIONS');  ------>代表支持的HTTP方法

//开户跨域访问
app.all('*',function (req, res, next) {
  res.header('Access-Control-Allow-Origin', '*');
  res.header('Access-Control-Allow-Headers', 'Accept,Content-Type,Content-Length, Authorization,X-Requested-With ');
  res.header('Access-Control-Allow-Methods', 'POST,GET,PUT,DELETE,OPTIONS');

  if ('OPTIONS' == req.method) {
    res.send(200); /让options请求快速返回/
  }
  else {
    next();
  }
});