Fix the route reversal helper to allow for empty paths/'home'-links

The route reversal helper doesnt account for

```
  match '', 'home#index', name: 'home'
```

This PR fixes that.

Note that there was already a failing test in the helper spec. I tried to clean it up but couldnt get it working, could someone take a look at it? Its the one checking for failed look ups that throws an exception.