COSMIT removed unused imports, fixed error message in test of boosting
test case for unfitted idf vector
raise ValueError for unfitted idf vector

Catches an obscure AttributeError raised by calling .transform on a TfidfTransformer (with use_idf=True) that has not learned an idf vector.
Merge pull request #1656 from rlmv/idf_diag

Fix AttributeError caused by an unlearned idf vector