Field types can change after being set to None

I find this behavior surprising with regards to the type safety ConfigDict offers:

```python
c = ConfigDict({'a': 1})
c.a = None
c.a = 'string'
```

I've managed to change the type of `a` from integer to string without any invocation of `ignore_type`. Would it perhaps make sense for the `a` field to "remember" that it was originally an int?

This is on the master branch (cd2928352989701659da17df7b9e649cfc2e6707).