I improved `IDFModel.transform` to allow using a single vector.

[[SPARK-4494] IDFModel.transform() add support for single vector - ASF JIRA](https://issues.apache.org/jira/browse/SPARK-4494)

Author: Yuu ISHIKAWA <yuu.ishikawa@gmail.com>

Closes #3603 from yu-iskw/idf and squashes the following commits:

256ff3d [Yuu ISHIKAWA] Fix typo
a3bf566 [Yuu ISHIKAWA] - Fix typo - Optimize import order - Aggregate the assertion tests - Modify `IDFModel.transform` API for pyspark
d25e49b [Yuu ISHIKAWA] Add the implementation of `IDFModel.transform` for a term frequency vector