[fix]Keeps inserting while a model instance has exists attribute

Changed private $data to $dataset (maybe even name it to $vocalDataset/$vocalData)
This prevents overwriting a relation with the same name (in my case).

When checking for an existing model, if someone already has model a instance with all its relations and data, there is an exists attribute which you can check against.

I tested it on my app, and it worked. Before merging it with the master branch check it a few times with your own app so that the old case also still works.