LDAP User synchronisation sometimes failed when the User document contains a Tag object

XWikiLDAPAuthServiceImpl.updateUserFromLDAP failed on XWiki.saveDocument when the user document contains a Tag Object. Symptoms are similar to XWIKI-2202.

See also XWIKI-2545 for the circumstances that make this happened. From that, just noticed that adding the tag object myself make this bug happen and removing it resolve the problem.

Be careful, that after the authentication has been lost due to the exception thrown by saveDocument, the user may login again, but any access to privileged action like Administration or Edition, cause him to be logged out again, with the same exception.