Improve storage update api

[Former user]

Allow for direct update without previous fetching so that this code example

with storage() as db:
    record = db.fetch(Todo, id, vsn)
    record.text = text
    record.done = done
    db.update(record)

can be simplified as follow:

with storage() as db:
    record = db.update(Todo, id, vsn, text = text, done = done)

[Marco Caramma]

Commit 8d95fc9