Fix for use of `published` field from different entities

In rare cases, activity was created at the end of a second, and an object at the beginning of the next second, so after formatting in RFC822, the activity and the object belonged to different seconds. Also in feed render was used object.data["published"], which was compared with activity.data["published"]. I've fixed feed tests.

Is it correct that the moment of creation of an activity and an object can differ in second after formatting?

Closes #1691 (closed)