FIX: Raise warnings in f_classif a given feature is constant across each class

Fixes https://github.com/scikit-learn/scikit-learn/issues/2359

This is because for each class, the variation within the samples is zero, which technically gives a F score of inf.
I think you got it wrong. It should raise a warning if a feature takes all the time the same value. So test should be with X[:, 0] = 2.

see original issue. Here feature is really informative.