Ensure min_count is passed through

Author: TomAugspurger

pandas/core/groupby.py

@@ -1237,6 +1237,8 @@ def groupby_function(name, alias, npfunc,
             def f(self, **kwargs):
                 if 'numeric_only' not in kwargs:
                     kwargs['numeric_only'] = numeric_only
+                if 'min_count' not in kwargs:
+                    kwargs['min_count'] = min_count
                 self._set_group_selection()
                 try:
                     return self._cython_agg_general(