Changed ActiveRecord::Relation#update behavior so that it will work on Relation objects without giving id

```ruby
#   Note: Updating a large number of records will run a
#   UPDATE query for each record, which may cause a performance
#   issue. So if it is not needed to run callbacks for each update, is
#   prefered to use <tt>update_all</tt> for updating all records using
#   a single query.
```