Add test cases for Migration#inverse_of

Adds test cases for check for the `ActiveRecord::IrreversibleMigration` for 
- `change_column`
- `change_column_default` 
- `change_column_null`
@matthewd here the idea is to make sure that the `change_column_null` will not raise `ActiveRecord::IrreversibleMigration`. I think I should add another test case which would tell about what will happen when I call `@recorder.inverse_of :change_column_null`. 
thoughts?