Field types can change after being set to None

[malmaud]

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

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 (cd29283).