Add passthrough option maxHeaderSize to node http.createServer()
To mitigate CVE-2018-12121 node maintainers made a breaking change in https://github.com/nodejs/node/commit/186035243fad247e3955fa0c202987cae99e82db that reduces header size from 80kb to 8kb. 

To remedy the fallout from this, a feature was added in node [v13.3.0](https://nodejs.org/api/http.html#http_http_createserver_options_requestlistener) that allows the maxHeaderSize option to be passed to http.createServer().

```javascript
http.createServer({
 maxHeaderSize: 8196*2
})
```

Koa does not currently support this. 

I have created a pull request to add the maxHeaderSize option to Koa. 

See also: 
[Limit on HTTP request headers size: #479](https://github.com/koajs/koa/issues/479)
https://github.com/nodejs/node/issues/24731
