set host header for proxied request

currently, this is wrong when the origin vhost we want has a different name
from the `hostname` we're configured to use.

### Example
 1. doorman is running at `private.host.com`, and is proxying requests back to `app.host.com`
 1. after auth/authz, the request to the origin server has a header `Host: private.host.com`
 1. the origin is unable to connect this request to the correct virtual host and serves the wrong content