it's String when we call `connection_handler.establish_connection method` with explicit connection_pool_name, e.g. here:
https://github.com/zendesk/active_record_shards/blob/master/lib/active_record_shards/connection_switcher.rb#L185

In other words:
```ruby
if __getobj__.is_a? String
  # assume it's connection_pool_name already
  __getobj__
else
  # assume it's ActiveRecord::Base object
  __getobj__.connection_pool_name
 end
```
I guess, ActiveRecordDecorator is a poor name and documentation is not 100% accurate, I'll change it ;)