default_box_create_on_get=False should still create keys on assigment

[wrdls]

As reported before in #164 (comment)

The current behavior of default_box_create_on_get is not what you would expect and eliminates a large part what makes default_box useful in the first place.

In the following case I would still expect b.foo.bar to be set to 1.

>>> b = Box(default_box=True, default_box_create_on_get=False)
>>> b.foo.bar
Box({})
>>> b.foo.bar = 1
>>> b.foo.bar
Box({})
>>> b
Box({})

Not sure if this is feasible, but a new config option default_box_create_on_set could be added to control this behavior.