Allow global override of default STI inheritance column

This change fixes a bug by which 3.2-STABLE users can't globally override the default STI inheritance column with `ActiveRecord::Base.inheritance_column = 'some_column'`. 3.2-STABLE users are forced to use a deprecated method or monkey patch it otherwise.

Test case written by tkhr <takehiro0740@gmail.com>.
Merge pull request #11019 from tkhr/test_case_for_issue_10393

Add test case for issue #10393 and fix warning message