By default, feed plugin should use the fetching date as entries creation date

Today when the feed plugin creates a feed entry document in the wiki, it tries to use the synd entry publish date metadata as the document creation date, thus "cheating" on the real document creation time. With this behavior, it's likely that the plugin will create document "from the past" (for example at dates when the wiki did not exist yet).

The entry real publishing date information is already made available in the associated XWiki.FeedEntryClass object, so the fetching date should be privileged as a default setting for feed entry document creation date (+ without doing that, that information is not available at all).

A config parameter will make the old behavior possible, for backward compatibility.