Fix schema dump works when non Oracle adapter used

This pull request addresses `NoMethodError undefined method` when `rake db:schema:dump` command executed for non Oracle database adapters, such as sqlite3 and postgresql.

As an exception, this pull request does not have unit tests but I already confirmed:
- No regression found in ActiveRecord and Oracle enhanced unit tests
- Tested by @stefanvr 

It should resolve issues reported #366 #407.