[FIX] base: don't swith back with su=False in _name_search

    When you have su == True in your env and you call _name_search
    the _name_search was switching back to su = False
    when you call with_user with the same user or with None

    To reproduce the issue, take a model where a user has
    no access.control.list allowing to read the model
    call name_search with sudo.