Fixed #14656 -- Added Atom1Feed `published` element

Some feed aggregators make use of the `published` element as well as
the `updated` element (within the Atom standard -- http://bit.ly/2YySb).

The standard allows for these two elements to be present in the same
entry. `Atom1Feed` had implemented the `updated` element which was
incorrectly taking the date from `pubdate`.