Fix VDestination bug that prevents setting custom "default" value

Fixes the bug where `VDestination`'s `default` parameter always gets overriden by `Validator`'s `__init__` method.

This change also indirectly fixes the bug that prevents `front.py`'s `GET_verify_email` method from redirecting browser to `/prefs/update` on success. (See https://github.com/reddit/reddit/blob/master/r2/r2/controllers/front.py#L1173)