Active Record Adapter: now respecting primary_key for loading models

This PR fixes a bug, when ActiveRecord Models have a different `primary_key` defined

Like so:

```ruby
class Person < ActiveRecord::Base
  self.primary_key = "person_id"
end
```

**IMPORTANT**: I need support generating a failing test-case before merging. I didn't have time looking into the testsuite, as this is a hotfix for an important project I'm currently working on - help appreciated!