BUG: fixes reversion of isnull/isna produced in merge

Author: deniederhut

Diff for: pandas/core/generic.py

@@ -26,7 +26,7 @@
     is_re_compilable,
     pandas_dtype)
 from pandas.core.dtypes.cast import maybe_promote, maybe_upcast_putmask
-from pandas.core.dtypes.missing import isnull, notnull
+from pandas.core.dtypes.missing import isna, notna
 from pandas.core.dtypes.generic import ABCSeries, ABCPanel, ABCDataFrame
 
 from pandas.core.common import (_values_from_object,