! Deserialization id setting bug fix

While working on our project we discovered a bug with setting the next_id after deserialization.
It appears that after the process the id is always count + 1 which will be wrong after deletion for example.

Steps to reproduce (in the console):
  1. Start the project
  2. `create` some models
      The `next_id` should now be `count + 1`
  3. `destroy` one (or more) of the models
      The `next_id` is still correct (i.e. does not change)
  4. Call `serialize_to_file`
      The `next_id` is still correct (i.e. does not change)
  5. `exit` the simulator
  6. Restart the app
      `next_id` is now `count + 1` which is not correct anymore

My guess this is because serialized data does not contain any information about the id column.

The easiest way we found to fix this problem is included in this pull request.