Currently LogisticRegressionWithLBFGS in python/pyspark/mllib/classification.py will invoke callMLlibFunc with a wrong "regType" parameter.
It was assigned to "str(regType)" which translate None(Python) to "None"(Java/Scala). The right way should be translate None(Python) to null(Java/Scala) just as what we did at LogisticRegressionWithSGD.

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #4831 from yanboliang/pyspark_classification and squashes the following commits:

12db65a [Yanbo Liang] correct LogisticRegressionWithLBFGS regType parameter for pyspark