Default ensure_ascii to False when writing notebook JSON

Alternative to gh-7064

IPython/nbformat/v4/nbjson.py

@@ -49,6 +49,7 b' class JSONWriter(NotebookWriter):'
         kwargs['indent'] = 1
         kwargs['sort_keys'] = True
         kwargs['separators'] = (',',': ')
+        kwargs.setdefault('ensure_ascii', False)
         # don't modify in-memory dict
         nb = copy.deepcopy(nb)
         if kwargs.pop('split_lines', True):