Set default root_object lazily.

I made this change to my own fork which didn't have 20bfde21e55ed2b4fcf147c6026f9e53ee177d84, because I was seeing `Engine#render` call `Helpers#data_name(default_object)` call `data.first` *twice* when `@whatevers` contains a collection, which would result in two unnecessary queries. This is no longer the case in master because of the mentioned commit, but I thought lazy setting of the default would be nicer anyway.