koa2跨域问题：@koa/cors 而非cors

koa2跨域，直接引用@koa/cors而非cors,@koa/cors版本更新到5.0，能解决跨域问题，用cors无论怎样都没用。

$ npm install @koa/cors --save

$ npm install @koa/cors --save

const Koa = require('koa');
const cors = require('@koa/cors');

const app = new Koa();
app.use(cors());

cors(options)

/**
 * CORS middleware
 *
 * @param {Object} [options]
 *  - {String|Function(ctx)} origin `Access-Control-Allow-Origin`, default is '*'
 *    If `credentials` set and return `true, the `origin` default value will set to the request `Origin` header
 *  - {String|Array} allowMethods `Access-Control-Allow-Methods`, default is 'GET,HEAD,PUT,POST,DELETE,PATCH'
 *  - {String|Array} exposeHeaders `Access-Control-Expose-Headers`
 *  - {String|Array} allowHeaders `Access-Control-Allow-Headers`
 *  - {String|Number} maxAge `Access-Control-Max-Age` in seconds
 *  - {Boolean|Function(ctx)} credentials `Access-Control-Allow-Credentials`, default is false.
 *  - {Boolean} keepHeadersOnError Add set headers to `err.header` if an error is thrown
 *  - {Boolean} secureContext `Cross-Origin-Opener-Policy` & `Cross-Origin-Embedder-Policy` headers.', default is false
 *  - {Boolean} privateNetworkAccess handle `Access-Control-Request-Private-Network` request by return `Access-Control-Allow-Private-Network`, default to false
 * @return {Function} cors middleware
 * @api public
 */

官方文档