Columns not marked as changed on instantiation

Is there a reason a column, set during instanciation, is not marked as changed, while it is when sets by an attribute ?

For example, in the following code, I expect the first call to `p1.get_changed_columns()` to returns `['last_name']`, as `p2` does:

```python
>>> from cqlengine import columns
>>> from cqlengine.models import Model
>>> class Person(Model):
...     first_name = columns.Text(primary_key=True)
...     last_name = columns.Text(default='something')
... 
>>> p1 = Person(first_name='bob', last_name='joe')
>>> p1.get_changed_columns()
[]
>>> p2 = Person(first_name='john')
>>> p2.get_changed_columns()
[]
>>> p2.last_name = 'james'
>>> p2.get_changed_columns()
['last_name']
>>> 
```

Is this a design choice or an issue ?