fix nose tests error

pandas.tseries.tests.test_resample.TestPeriodIndex
and
pandas.tests.test_groupby.test_timegrouper_with_reg_groups
need default dropna param in _cython_agg_general()

Author: OXPHOS

pandas/core/groupby.py

@@ -794,7 +794,7 @@ def _cython_transform(self, how, numeric_only=True):
 
         return self._wrap_transformed_output(output, names)
 
-    def _cython_agg_general(self, how, numeric_only=True):
+    def _cython_agg_general(self, how, numeric_only=True, dropna=True):
         output = {}
         for name, obj in self._iterate_slices():
             is_numeric = is_numeric_dtype(obj.dtype)