Setting response.expires via timedelta produces an incorrect header value

[mschmitzer]

When using a timedelta to calculate the absolute timestamp, webob uses datetime.now(), which is in local time. This is later re-interpreted to be UTC, which yields an "Expires" header value that is off by the UTC offset of the local timezone.

Example in CEST:

Python 3.8.5 (default, Jan 27 2021, 15:41:15) 
[GCC 9.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from datetime import datetime, timedelta
>>> from webob import Response
>>> datetime.utcnow()
datetime.datetime(2021, 4, 23, 13, 48, 1, 541647)
>>> r = Response()
>>> r.expires = timedelta(seconds=0)
>>> r.expires
datetime.datetime(2021, 4, 23, 15, 48, 13, tzinfo=UTC)

The example shows that setting expires to a zero timedelta produces a header value two hours in the future.

The cache_expires method does the right when passed a timedelta.