FIX #2443: _limit_features now keeps correct top features when max_features is not None

A fix for issue #2443.
- I have added a regression test for CountVectorizer but I am not sure if there should be another one for TfidfVectorizer. Perhaps someone more well-versed with tf-idf should look into this.
- I could not run the full tests due to (I think) the OpenBLAS issue, but tests for sklearn.feature_extraction are successful.