add defaults to instance._data, fix a bug with ListField

ie.
class TestSchem(DocumentSchema):
    a = TextField()

class TestDoc(Document):
    test = ListField("TestSchem")

t = TestDoc.wrap({})
t.test.append(TestSchem(a="test"))

print t.test
print t.unwrap()

before:
< []
< {'test': []}

after:
< [{'a': u'test'}]
< {'test': [{'a': u'test'}]}