Correctly encode aware, non-UTC datetime objects

http_date() requires timetuple in UTC, but JSONEncoder.default() was
passing a local timetuple instead.

CHANGES

@@ -79,6 +79,7 @@ Major release, unreleased
 - Removed error handler caching because it caused unexpected results for some
   exception inheritance hierarchies. Register handlers explicitly for each
   exception if you don't want to traverse the MRO. (`#2362`_)
+- Fix incorrect JSON encoding of aware, non-UTC datetimes. (`#2374`_)
 
 .. _#1489: https://github.com/pallets/flask/pull/1489
 .. _#1621: https://github.com/pallets/flask/pull/1621
@@ -102,6 +103,7 @@ Major release, unreleased
 .. _#2354: https://github.com/pallets/flask/pull/2354
 .. _#2358: https://github.com/pallets/flask/pull/2358
 .. _#2362: https://github.com/pallets/flask/pull/2362
+.. _#2374: https://github.com/pallets/flask/pull/2374
 
 Version 0.12.2
 --------------

flask/json/__init__.py

@@ -1,7 +1,7 @@
 # -*- coding: utf-8 -*-
 import io
 import uuid
-from datetime import date
+from datetime import date, datetime
 from flask.globals import current_app, request
 from flask._compat import text_type, PY2
 
@@ -62,6 +62,8 @@ class JSONEncoder(_json.JSONEncoder):
                     return list(iterable)
                 return JSONEncoder.default(self, o)
         """
+        if isinstance(o, datetime):
+            return http_date(o.utctimetuple())
         if isinstance(o, date):
             return http_date(o.timetuple())
         if isinstance(o, uuid.UUID):