old_contents: 
  var koa = require('koa');
  var charset = require('./');
  
  var app = koa();
  
  app.use(charset());
  app.use(function* () {
    this.body = '你好！';
    this.type = 'text/html; charset=gbk';
  });
  
  app.listen(3000);
  
new_contents: const Koa = require('koa');
  const charset = require('.');
  
  const app = new Koa();
  
  app.use(charset());
  app.use(function (ctx) {
    ctx.body = 'Hello World！';
    ctx.type = 'text/html; charset=gbk';
  });
  
  app.listen(3000);
subject: update example with koa@2.x.x ✨ ..