Don't try to get the subclass if the inheritance column doesn't exist

The `subclass_from_attrs` method is called even if the column specified by
the `inheritance_column` setting doesn't exist. This prevents setting associations
via the attributes hash if the association name clashes with the value of the setting,
typically `:type`. This worked previously in Rails 3.2.
Merge pull request #13710 from ujjwalt/hotfix/sti

Active Record's subclass_from_attributes shouldn't assume :type is for STI unless there is a type column.