Duplicate column_defaults properly

Backport c517602 to fix #6115

Deleted:
    activerecord/lib/active_record/core.rb

Conflicts:
    activerecord/test/cases/base_test.rb
This pull requests addresses ORA-01400 errors and also supports #6115 issue tested.

Issue #6115 has been fixed and tested with
the attribute `:null => false, :default => ""`
However `:null => false` attribute is not necessary to test this issue,
which causes many ORA-01400 errors with Oracle enhanced adapter.