COSMIT: better notation in CountVectorizer.
COSMIT: same changes in transform method.
COSMIT: more robust condition in inverse_transform.

We need to support other formats than CSC and COO. Besides, tocsr()
doesn't trigger a copy if the matrix is already CSR.
FIX typo in test for RdigeCV
ENH Use csr_matrix.sum_duplicates instead of tocoo