Fix changeset method for multi-flush transactions

[oinopion]

When in the same transaction you create object, flush, update object and commit transaction, Continuum will first flush insert version, but later it will update it to be update (happening here: https://github.com/kvesteri/sqlalchemy-continuum/blob/master/sqlalchemy_continuum/unit_of_work.py#L168)

This causes changeset to return empty dict for such changes, which is incorrect. I can see two solutions:

1. Change behaviour of changeset to not ignore such versions
2. Change behaviour of process_operation to not change insert into update (explored in Versioning issues on session.flush #79)

This PR changes behaviour of changeset method to not ignore changes on update versions without previous versions.

This fixes #141.

[netcriptus]

@kvesteri any chance this can be merged to master anytime soon now? I'm struggling with the same problem, and this PR solves it.

[kvesteri]

Thanks for the PR! 🚋

[oinopion]

🙌