Correct missing keys
closes #3142 

The problem came from the fact that `from_pretrained` set the model on which to load the weights to the base model: it detected that the state dict was made for the base, therefore loading only onto the base.

It didn't look for the weights it didn't load. Here, both state dicts are analyzed and the difference (keys present in the state dict with the head and not in the base state dict) are added to the missing keys.

Added a test.
Very good catch