Fix to work the example code

```
% node index.js

/private/var/folders/3s/cl_mjsfx7kl941cvnp75h0qcg5kd_k/T/tako/index.js:11
    this.addHeader('X-Powered-By', 'Node.js')
         ^
TypeError: Object #<ServerResponse> has no method 'addHeader'
    at ServerResponse.addPoweredBy (/private/var/folders/3s/cl_mjsfx7kl941cvnp75h0qcg5kd_k/T/tako/index.js:11:10)
    at ServerResponse.writeHead (/private/var/folders/3s/cl_mjsfx7kl941cvnp75h0qcg5kd_k/T/node_modules/on-headers/index.js:44:16)
    at ServerResponse._implicitHeader (http.js:1129:8)
    at ServerResponse.OutgoingMessage.end (http.js:918:10)
    at Server.onRequest (/private/var/folders/3s/cl_mjsfx7kl941cvnp75h0qcg5kd_k/T/tako/index.js:19:7)
    at Server.emit (events.js:98:17)
    at HTTPParser.parser.onIncoming (http.js:2108:12)
    at HTTPParser.parserOnHeadersComplete [as onHeadersComplete] (http.js:121:23)
    at Socket.socket.ondata (http.js:1966:22)
    at TCP.onread (net.js:527:27)
```