[SPARK-15009][PYTHON][FOLLOWUP] Add default param checks for CountVectorizerModel

## What changes were proposed in this pull request?

Adding test for default params for `CountVectorizerModel` constructed from vocabulary.  This required that the param `maxDF` be added, which was done in SPARK-23615.

## How was this patch tested?

Added an explicit test for CountVectorizerModel in DefaultValuesTests.

Author: Bryan Cutler <cutlerb@gmail.com>

Closes apache#20942 from BryanCutler/pyspark-CountVectorizerModel-default-param-test-SPARK-15009.

Author: BryanCutler

python/pyspark/ml/tests.py

@@ -2096,6 +2096,11 @@ def test_java_params(self):
                     # NOTE: disable check_params_exist until there is parity with Scala API
                     ParamTests.check_params(self, cls(), check_params_exist=False)
 
+        # Additional classes that need explicit construction
+        from pyspark.ml.feature import CountVectorizerModel
+        ParamTests.check_params(self, CountVectorizerModel.from_vocabulary(['a'], 'input'),
+                                check_params_exist=False)
+
 
 def _squared_distance(a, b):
     if isinstance(a, Vector):