set the Host on redirect

Fixes #7.

```
$ curl http://myapp-staging.herokuapp.com/ -i
HTTP/1.1 301 Moved Permanently
Content-Type: text/html; charset=utf-8
Date: Thu, 24 Oct 2013 23:05:20 GMT
Location: https://myapp-staging.herokuapp.com/
Content-Length: 72
Connection: keep-alive

<a href="https://myapp-staging.herokuapp.com/">Moved Permanently</a>.
```