inserting null on an autoincrement primary key prevents cleanup

When I have a table with a primary key column that's also increment, it's valid to set it to `null` on the insert statement. the last insert it will then return the auto increment value, which is the primary key in that case.

As already noticed in #44, the auto increment column doesn't have to be part of the primary key, so the return value of last insert id ironically doesn't necessarily belong to the primary key. However, #44 also broke the case, which one might perceive as more common, that the last insert id may also be the primary key.

So, `haveInDatabase(table, ['id' => null, ...])` with id as autoincrement and pk will lead to the id being set to `null` for cleanup, which results in the row not being removed at teardown.

I have prepared a fix for this case, which hopefully is specific enough to not interfere with other use cases.

首先使用自增主键复现 haveInDatabase(table, ['id' => null, ...]) 的情况，然后跟踪插入的 ID 是如何为 teardown cleanup 记录的。完成的标准是保留生成的主键值，并在 cleanup 期间删除插入的行，同时不破坏自增列不是主键的情况。

由索引模型根据 Issue 内容生成。