Default ensure_ascii to False when writing notebook JSON

Alternative to gh-7064

IPython/nbformat/v4/nbjson.py

@@ -1,67 +1,68 b''
 """Read and write notebooks in JSON format."""
 
 # Copyright (c) IPython Development Team.
 # Distributed under the terms of the Modified BSD License.
 
 import copy
 import json
 
 from IPython.utils import py3compat
 
 from .nbbase import from_dict
 from .rwbase import (
     NotebookReader, NotebookWriter, rejoin_lines, split_lines, strip_transient
 )
 
 
 class BytesEncoder(json.JSONEncoder):
     """A JSON encoder that accepts b64 (and other *ascii*) bytestrings."""
     def default(self, obj):
         if isinstance(obj, bytes):
             return obj.decode('ascii')
         return json.JSONEncoder.default(self, obj)
 
 
 class JSONReader(NotebookReader):
 
     def reads(self, s, **kwargs):
         """Read a JSON string into a Notebook object"""
         nb = json.loads(s, **kwargs)
         nb = self.to_notebook(nb, **kwargs)
         return nb
 
     def to_notebook(self, d, **kwargs):
         """Convert a disk-format notebook dict to in-memory NotebookNode
         
         handles multi-line values as strings, scrubbing of transient values, etc.
         """
         nb = from_dict(d)
         nb = rejoin_lines(nb)
         nb = strip_transient(nb)
         return nb
 
 
 class JSONWriter(NotebookWriter):
 
     def writes(self, nb, **kwargs):
         """Serialize a NotebookNode object as a JSON string"""
         kwargs['cls'] = BytesEncoder
         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):
             nb = split_lines(nb)
         nb = strip_transient(nb)
         return py3compat.str_to_unicode(json.dumps(nb, **kwargs), 'utf-8')
 
 
 _reader = JSONReader()
 _writer = JSONWriter()
 
 reads = _reader.reads
 read = _reader.read
 to_notebook = _reader.to_notebook
 write = _writer.write
 writes = _writer.writes