Make JSONEncoder keep the same type for date/datetime.

Otherwise Celery jobs start to get `datetime` in place of `date` and that
could lead to errors.

See celery/celery#7754, related PR celery#1515.

kombu/utils/json.py

@@ -38,10 +38,13 @@ def default(self, o,
             return reducer()
         else:
             if isinstance(o, dates):
+                marker = "__date__"
                 if not isinstance(o, datetime):
                     o = datetime(o.year, o.month, o.day, 0, 0, 0, 0)
+                else:
+                    marker = "__datetime__"
                 r = o.isoformat()
-                return {"datetime": r, "__datetime__": True}
+                return {"datetime": r, marker: True}
             elif isinstance(o, times):
                 return o.isoformat()
             elif isinstance(o, uuid.UUID):
@@ -71,6 +74,8 @@ def dumps(s, _dumps=json.dumps, cls=None, default_kwargs=None, **kwargs):
 
 def object_hook(dct):
     """Hook function to perform custom deserialization."""
+    if "__date__" in dct:
+        return datetime.datetime.fromisoformat(dct["datetime"]).date()
     if "__datetime__" in dct:
         return datetime.datetime.fromisoformat(dct["datetime"])
     if "__bytes__" in dct:

t/unit/utils/test_json.py

@@ -2,7 +2,7 @@
 
 import uuid
 from collections import namedtuple
-from datetime import datetime
+from datetime import date, datetime
 from decimal import Decimal
 from unittest.mock import MagicMock, Mock
 
@@ -39,7 +39,7 @@ def test_datetime(self):
             'datetime': now,
             'tz': now_utc,
             'time': now.time().isoformat(),
-            'date':  datetime(now.year, now.month, now.day, 0, 0, 0, 0),
+            'date':  date(now.year, now.month, now.day),
         }
 
     @given(message=st.binary())