Improve Naive Bayes test coverage

This PR adds tests for the binarize=None option in BernoulliNB. If binarize=None, and non-binary data is used, an error should be raised (which is now the case.)

Test coverage has been improved in f6bf82f, by removing a conditional that was always true (the check_arrays call above serves the same purpose.)

Coverage is now 99% for naive_bayes.py. it should be 100%, but the @abstractmethod decorator on line 39 is incorrectly counted as a missing branch due to a bug in coverage.py (see https://bitbucket.org/ned/coveragepy/issue/129/misleading-branch-coverage-of-empty for details.)