Allow 'defaults' override on nested dataclasses

[jacebrowning]

There's a test demonstrating the expected behavior here:

datafiles/tests/test_setup.py

Lines 106 to 112 in 7977594

	@pytest.mark.xfail(reason='https://github.com/jacebrowning/datafiles/issues/64')
	def it_converts_attributes(expect, sample):
	expect(sample.datafile.data) == {
	'name': "a",
	'score': 0.5,
	'nested': {'name': "b"},
	}