Just in case they get mutated after a write action is added to the queue:
```python
Model.start_queue()
record = Model()
record.save()  # The `insert` action is enqueued
record.data['foo'] = 'bar'  # Potentially mutates the saved data
Model.commit_queue()
```
The commit should save the data that was enqueued, not the data that was changed after `save` was called.

Now that you mention it, though, I see that handling mutation weirdness is already the responsibility of `Field::to_storage`--mutable values are deep-copied when they get serialized. So yeah, we shouldn't need the copy here.