Column names weren't checked for uniqueness for subqueries.
Code for names uniqueness checking used for view creation moved into
separate function named check_duplicate_names(). It's called on
preparation of subqueries to check uniqueness of names. If duplicate names
are found then error is raised.
now default behaviour do not prevent any updates of view.
Also updating VIEWs of queries without LIMIT clause will not be checked at all (there will not be even warning)
(changes according to discussion on last dev-conf)
