[SPARK-6080] [PySpark] correct LogisticRegressionWithLBFGS regType parameter for pyspark

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

python/pyspark/mllib/classification.py

@@ -207,7 +207,7 @@ class LogisticRegressionWithLBFGS(object):
         """
         def train(rdd, i):
             return callMLlibFunc("trainLogisticRegressionModelWithLBFGS", rdd, int(iterations), i,
-                                 float(regParam), str(regType), bool(intercept), int(corrections),
+                                 float(regParam), regType, bool(intercept), int(corrections),
                                  float(tolerance))
 
         return _regression_train_wrapper(train, LogisticRegressionModel, data, initialWeights)