[SPARK-25959][ML] GBTClassifier picks wrong impurity stats on loading

Author: mgaido91

mllib/src/main/scala/org/apache/spark/ml/tree/treeParams.scala

@@ -538,7 +538,7 @@ private[ml] object GBTClassifierParams {
     Array("logistic").map(_.toLowerCase(Locale.ROOT))
 }
 
-private[ml] trait GBTClassifierParams extends GBTParams with TreeClassifierParams {
+private[ml] trait GBTClassifierParams extends GBTParams with TreeRegressorParams {
 
   /**
    * Loss function which GBT tries to minimize. (case-insensitive)

mllib/src/test/scala/org/apache/spark/ml/classification/GBTClassifierSuite.scala

@@ -448,6 +448,7 @@ class GBTClassifierSuite extends MLTest with DefaultReadWriteTest {
         model2: GBTClassificationModel): Unit = {
       TreeTests.checkEqual(model, model2)
       assert(model.numFeatures === model2.numFeatures)
+      assert(model.featureImportances == model2.featureImportances)
     }
 
     val gbt = new GBTClassifier()