fix: adding frozen boxes

[m-janicki]

Fix #291

I'm not entirely convinced this solution is valid. Maybe the merge_update function should accept a frozen_merge keyword argument. This argument would be set to True in the __add__ method, allowing all the new code to be conditional within an if statement?

[cdgriffith]

Suggested change

	frozen = self[k]._box_config['frozen_box']
	self[k]._box_config['frozen_box'] = False
	self[k].merge_update(v, box_merge_lists=merge_type)
	self[k]._box_config['frozen_box'] = frozen
	self[k]._box_config["frozen_box"] = self._box_config["frozen_box"]
	self[k].merge_update(v, box_merge_lists=merge_type)

I think it might make more sense to just set it to the parent's setting, so it should always be as expected.

[m-janicki]

That makes sense, but I'm afraid it breaks frozen attribute for nested data. I've added 2 more test cases, ran this code against them, but one of them failed

Box/test/test_box.py

Lines 999 to 1004 in 4f3a979

	def test_adding_nested_frozen_boxes_result_in_frozen_box(self):
	a = Box({'one': {"two": '1.2'} }, frozen_box=True)
	b = Box({'one': {"three": '1.3'} }, frozen_box=True)
	c = a + b
	with pytest.raises(BoxError):
	c.one.four = '1.4'

I think here is what happens:

1. In __add__ method frozen is temporarily set to False (see code)
2. Later, when running self[k]._box_config["frozen_box"] = self._box_config["frozen_box"] The frozen = False is assigned to the nested box, and it is never set to True.