[MRG] FIX for exception thrown when y does not have all classes

This is a fix for #3581
I remember that if one of the `classes` is not present in `y` , the count was set to one, before it was changed in this PR. https://github.com/scikit-learn/scikit-learn/pull/2876/ . you might need to do

    recip_freq[recip_freq == 0] = 1

However I am not sure how much that is right, ping @larsmans ?