fix(ngModel): test & update correct model when running $validate

If `$validate` is invoked when the model is already invalid, `$validate` should pass `$$invalidModelValue` to the validators, not `$modelValue`. More over, if the model has become valid, the previously invalid model should be assigned to `$modelValue`. Lastly, in case the model has become invalid, the previously valid model should be assigned to `$$invalidModelValue`.

Closes #7836
sorry, thought this is a different test. I mean that after the preparation I'm changing the `max` to be `6` and seeing that the previously entered value of `12345` is now on the scope (since now it is valid)