Different rounding for timestamp and datetime when set last_modified

[serhiy-storchaka]

Setting the last_modified property of StreamResponse rounds the value to seconds if it is float or datetime. But different rounding mode is used for float or datetime. If it is a float (timestamp), it is rounded toward +infinity. If it is an instance of datetime, it is rounded toward -infinity.

For example:

$ cat test_last_modified.py
from aiohttp.web import StreamResponse
from datetime import datetime, timezone

resp = StreamResponse()
dt = datetime.now(timezone.utc)
resp.last_modified = dt
print(resp.last_modified)
print(repr(resp.last_modified))
print(resp.headers["Last-Modified"])
resp.last_modified = dt.timestamp()
print(resp.last_modified)
print(repr(resp.last_modified))
print(resp.headers["Last-Modified"])

$ python test_last_modified.py
2020-12-02 09:51:02+00:00
datetime.datetime(2020, 12, 2, 9, 51, 2, tzinfo=datetime.timezone.utc)
Wed, 02 Dec 2020 09:51:02 GMT
2020-12-02 09:51:03+00:00
datetime.datetime(2020, 12, 2, 9, 51, 3, tzinfo=datetime.timezone.utc)
Wed, 02 Dec 2020 09:51:03 GMT

I don't know what direction is correct, but it should be the same.