finish handler

```javascript
var app = require('koa')();
var wechat = require('../lib/wechat');

var config = {
  path: '/wx/api',
  token: 'feit'
};

app.use(wechat(config).all(function *() {
  console.log(this.weixin.Content);

}).text(function *() {
  this.body = this.weixin.Content;

}).middlewarify());

app.listen(3000, function () {
  console.log('wechat server start!');
});
```
可能还要加上`this.type = 'application/xml';`