* test_state: augment modify_state test for writing MutableProxy
If the object contains a MutableProxy inside of it, then we get a pickling
error.
* Implement __reduce_ex__ for MutableProxy
Pass through `__reduce_ex__` onto the wrapped instance to strip it off when
cloudpickling to redis.
* base: get_value actually works with a str key
Unless the key isn't a field on the model, then it falls back to the previous
behavior of just returning the given key as is... why does it do this? I don't
know.
