Protocol relative URLs missing port in Rails URL helpers

I'm trying to generate protocol relative URLs using Rails URL helpers. For this I set the protocol option to false. But whenever I generate a URL, the port part is missing.

```
dashboard (master) > rails c
Loading development environment (Rails 4.0.3)
2.0.0p247 :001 > include Rails.application.routes.url_helpers
 => Object
2.0.0p247 :002 > tunnels_url(host: "0.0.0.0:3000", protocol: false)
 => "//0.0.0.0/tunnels"
```

The thing is: I'd like to generate protocol relative URL but using a custom port.