specify the seed to randomforestregressor test

Author: holdenk

python/pyspark/ml/regression.py

@@ -254,7 +254,7 @@ class RandomForestRegressor(JavaEstimator, HasFeaturesCol, HasLabelCol, HasPredi
     >>> df = sqlContext.createDataFrame([
     ...     (1.0, Vectors.dense(1.0)),
     ...     (0.0, Vectors.sparse(1, [], []))], ["label", "features"])
-    >>> rf = RandomForestRegressor(numTrees=2, maxDepth=2)
+    >>> rf = RandomForestRegressor(numTrees=2, maxDepth=2, seed=42)
     >>> model = rf.fit(df)
     >>> test0 = sqlContext.createDataFrame([(Vectors.dense(-1.0),)], ["features"])
     >>> model.transform(test0).head().prediction