FormAndSave: model update should happen later

Model update should happen not in initialization moment, but later.
Otherwise we can get situation, that model is not yet initialized when f->update() executes.

For example,
$model = $page->add('Model_Foo');
$form = $page->add('FormAndSave');
$form->setModel($model);
submitting will not work because model is associated to form after form initialization.
This happens at least in situation when we use Model with Session or Array controller.