XWiki.XWikiPreference content author is always XWiki.XWikiGuest

I think that's because of XWIKI-5503.

It modify the content author of the context document to drop PR. Problem is that it's modify content author of the XWikiDocument in cache which mean that it produce an inconsistency between cache and database and make impossible to change XWikiPreference content author since it reset it all the time.

It's also probably true for any other application that use this feature with another document as context document.