fixed failed test

Author: brkyvz

python/pyspark/sql/dataframe.py

@@ -887,7 +887,7 @@ def cov(self, col1, col2):
             raise ValueError("col1 should be a string.")
         if not isinstance(col2, str):
             raise ValueError("col2 should be a string.")
-        return self.df._jdf.stat().cov(col1, col2)
+        return self._jdf.stat().cov(col1, col2)
 
     @ignore_unicode_prefix
     def withColumn(self, colName, col):
@@ -1340,7 +1340,7 @@ def __init__(self, df):
         self.df = df
 
     def cov(self, col1, col2):
-        return self.stat.cov(col1, col2)
+        return self.df.cov(col1, col2)
 
     cov.__doc__ = DataFrame.cov.__doc__