Fix List and Dict valid_data

[lafrech]

I've been trying to fix a few issues about valid_data in List and Dict.

Fixes #766.

[lafrech]

A few words of explanation.

In List field, I applied the changes described in #766.

Since it is used in a Dict test, I ended up modfying both fields in the same PR.

In Dict field, I passed the valid data as valid_data kwarg rather than data (bugfix), and I modified the logic to ensure valid data contains items for which the key deserializes correctly (even if value deserializes correctly, it is worthless if the key is incorrect) and the value deserializes at least partially (valid_data is not None). I think it makes for a more useful valid data structure.

In the code before this change, the keys array is initialized with serialized keys and then each key is replaced with its deserialized version, but in case of ValidationError, the serialized key remains in the result dict.

One thing worth mentioning, and I noted it in a comment: [|de]serializetion outputs a simple dict whatever the input dict type. We could allow specifying the dict type, or even use a trick to return the same type (I'm thinking of using copy and clear, but that sounds terribly hackish).

[lafrech]

Just reviewed and removed the WIP in title as I think this is good to go.