fix to_datetime() when given timezone naive datetime input

[Christian Boos]

While looking at some specifics of the trac.util.datefmt.to_datetime function, I discovered that a particular usage pattern is problematic with pytz timezones:

>>> from trac.util.datefmt import *
>>> from datetime import datetime
>>> t = to_datetime(datetime(2012,1,1), timezone("Europe/Paris"))
>>> t.utcoffset()
datetime.timedelta(0, 540)

(from ​gmessage:trac-dev:V0HeDyfa_pk/5r33UatF3nEJ)

The utcoffset() plays a role when converting a time to the number of ms (to_utimestamp()). Here, it has a value of 9 minutes, which is quite wrong for our purpose.

The workaround is to always specify a timezone aware datetime object to to_datetime, but this is not documented, and shouldn't be necessary either (assuming a timezone naive datetime should actually be the same as a datetime in utc).

The following should

trac/util/datefmt.py

@@ -66 +66 @@
     elif isinstance(t, datetime):
         if tzinfo is not None:
             if t.tzinfo is None:
-                t = t.replace(tzinfo=tzinfo)
+                t = tzinfo.localize(t)
             else:
                 t = tzinfo.normalize(t.astimezone(tzinfo))
         return t

With this change, the t.utcoffset() for the example above would give 3600 seconds, i.e. the one hour we would expect.