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.
yes it is informative, no doubt. but a setting like this leads to a F score of infinity, right?

Should we not raise a warning for this also? That explains the warning message "The given data has features that are constant for each class."

If you think that this should be not the case, then I'll raise a warning only for the case where a features takes the same value all the time.