failing spec demonstraing not working delete command

I was following http://rom-rb.org/tutorials/rails/ tutorial and found issue when trying to implement `destroy` action in my rails app.

Running: `users.try { delete(:by_name, 'Piotr') }`
throws:
```
NoMethodError:
  undefined method `size' for #<#<Class:0x007fd82aa522b0>:0x007fd82d3f7d68>
```

I don't know if this is bug in rom-rails or in rom, but definitely when fixed it would be nice to have it covered here.
Few of them errored with https://travis-ci.org/rom-rb/rom-rails/jobs/45608784
Looks like they depend on order and assigned id from database.