Preserve Box subclass for nested boxes after unpickling

[koriyoshi2041]

Fixes #308.

A Box subclass is lost on nested attribute access after unpickling: nested boxes come back as the base box.box.Box instead of the subclass.

Box.__init__ sets box_class from self.__class__:

# __init__
"box_class": box_class if box_class is not None else self.__class__,

but Box.__new__ hardcodes the base Box:

# __new__
"box_class": box_class if box_class is not None else Box,

Unpickling calls __new__ + __setstate__, not __init__, so a subclass instance is restored with box_class=Box. Nested dicts converted to boxes on access (conversion box) then use box_class, producing base Box objects. Using cls in __new__ (which is the actual subclass) matches __init__.

Added test_pickle_preserves_subclass_on_nested_access: pickles a Box subclass with a nested dict and asserts the nested box is still the subclass after a round-trip. It fails on master (nested box is base Box) and passes with the fix; existing pickle tests still pass. (The subclass is defined at module level so its instances are picklable.)