[SPARK-4494][mllib] IDFModel.transform() add support for single vector

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)
Sorry, fixed it
https://github.com/yu-iskw/spark/commit/256ff3d9532cb209316c984c0e642cdbc83ccd7c#diff-7c5eb57aa2d7d6da7afb24b85429ac14L181