Increment to next sequence number even if model is not new

[slamuu]

Bug, feature request, or proposal:

proposal

What is the expected behavior?

If the model is not new but auto-incremented field is undefined, create a new field that represents the next incrementation

What is the current behavior?

The incremented field is only created if the model is new

What is the use-case or motivation for changing an existing behavior?

There is no way to easily regenerate to the next sequence number. The use case was related to #4, which required me to generate new sequence number for the duplicates