Make default values available on instantiation

On instantiation, default column values are not available. See:

```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')
>>> p1.first_name
'bob'
>>> p1.last_name
>>> 
```

Here I expect `p1.last_name` to return `'something'`, but it's not the case. This PR fix this issue.