Fix discrepancy when setting attributes when creating and updating

- `book.data['title'] = nil` sets the key 'title' in the `data` column
  during creation
- `book.data['title'] = 'something else'` does not properly overwrite
  'title'

Since we need to always set the key during creation (if a book was
created before with no default title, but the code was later changed to
have a default title, we don't want the titles automatically
defaulting), and to also properly set it during updating, then use both
formats:

- `book.data['title'] = 'something else'`, and...
- `book.write_store_attribute :data, :title, 'something else'`
Prepare 0.1.3