[MRG+1] Support unseen labels LabelBinarizer and test (Backward compatibility)

Patches the updates to LabelBinarizer for sparse support which broke the case the case with unseen labels. For issue #3462
I don't think you need `y_seen_ind` to calculate the `indptr`. I think `indptr` is just `np.hstack(0, np.cumsum(~y_in_classes))`