fix manager.get (and/or allow partial save)

[timbu]

def test_save_with_placeholders(bound_manager):
    manager = bound_manager

    manager.save(CustomFieldsEntity)
    entity = CustomFieldsEntity(id=1, name="foo")
    manager.save(entity)

    loaded = manager.get(CustomFieldsEntity, id=1)
    data = manager.serialize(loaded)
    assert isinstance(data['name'], TranslatableString)

    loaded.extra1 = "value1"
    manager.save(loaded)

    reloaded = manager.get(CustomFieldsEntity, id=1)
    redata = manager.serialize(reloaded)
    assert isinstance(redata['name'], TranslatableString)
    assert isinstance(redata['extra1'], TranslatableString)

    translator = get_translator(manager)
    translated = translator.translate(redata)
    assert translated['name'] == 'foo'
    assert translated['extra1'] == 'value1'