Status is set to None after refresh signature

[leplatrem]

By looking at the tracking-protection-lists collection history, I can see that the status oscillates between null and to-resign.
Same for normandy-recipes-capabilities.

This is not the case of other collections.

There is something wrong about this code: https://github.com/Kinto/kinto-signer/blob/master/kinto_signer/listeners.py#L187-L191
If the old status is None, we should set to signed and not None`.