Manually setting an value of a key of a Config through `__setitem__` invalidates it's `_values`

[eddiebergman]

It does not seem to make sense that manually setting an item with config[key] = value should delete the entire attribute self._values. Looking through the rest of the code, it seems like this would break a lot of functionality.

[mfeurer]

Looking through the rest of the code, it seems like this would break a lot of functionality.

Could you please provide further details here? I don't think we had any issues on this end yet.

[eddiebergman]

Just to document, after our discussion, we had a thought to make Configuration objects immutable to saveon validation time as we can guarantee validaty at construction time, rather than everytime it's queried.