Fixes ovr in the binary classifier case

Fixes https://github.com/scikit-learn/scikit-learn/issues/3700. Also allows feature arrays (rather than just numpy arrays) in the multiclass case (already supported in the binary case), and adds a test for the number of probabilities returned from predict_proba in test_ovr_binary.
It's actually the wrong way around, but I just pushed this commit and a fix to master. Thanks @wlamond!