Setting config.user_attributes doesn't seem to pick up other attributes

Setting custom user attributes to send to Airbrake about our current user do not seem to work.

```ruby
# in config/initializers/airbrake.rb
Airbrake.configure do |config|
  config.api_key = 'API_KEY'
  config.user_attributes = [:class_name, :id, :email]
end
```

```ruby
class ApplicationController < ActionController::Base
  def current_user
    current_agent || current_customer
  end
  helper_method :current_user
end
```

```ruby
# app/models/customer.rb
class Customer < ActiveRecord::Base
  def class_name; self.class.name; end
end

# app/models/agent.rb
class Agent < ActiveRecord::Base
  def class_name; self.class.name; end
end
```
Resulting exception displayed in Airbrake UI:

![Screen Shot 2013-03-20 at 10 36 20 AM](https://f.cloud.github.com/assets/26285/281978/bdeb9b52-9184-11e2-88c0-14f0ef398a40.png)