Added tests for deletion of columns on migration

- Rewrote The description of two tests
- Added two tests for deletionwhen Db:create is called and rows are removed
Added column deletion on Db:create

Db:create will now delete columns, if they are present in the current database,
but not in the new schema. All data in said columns will be lost. The change was
necessary as former versions of the library added options as columns, which was
rectified but made existing databases unreadable to the library.