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.

(cherry picked from commit e8d1d84837a59ef7d73b29b16ee05cd610d30a90)

Conflicts:
	activerecord/CHANGELOG.md
	activerecord/test/schema/schema.rb