[FIX] fields: timezone aware context_timezone

context_timestamp should always return a timezone aware timestamp, even when
no timezone is set on the user.
7f88681 fixed the bug in old api fields (openerp/osv/fields.py) but it was not
applied to new api fields (openerp/fields.py). opw 616612

openerp/fields.py

@@ -1242,17 +1242,16 @@ class Datetime(Field):
         """
         assert isinstance(timestamp, datetime), 'Datetime instance expected'
         tz_name = record._context.get('tz') or record.env.user.tz
+        utc_timestamp = pytz.utc.localize(timestamp, is_dst=False)  # UTC = no DST
         if tz_name:
             try:
-                utc = pytz.timezone('UTC')
                 context_tz = pytz.timezone(tz_name)
-                utc_timestamp = utc.localize(timestamp, is_dst=False)  # UTC = no DST
                 return utc_timestamp.astimezone(context_tz)
             except Exception:
                 _logger.debug("failed to compute context/client-specific timestamp, "
                               "using the UTC value",
                               exc_info=True)
-        return timestamp
+        return utc_timestamp
 
     @staticmethod
     def from_string(value):