Explicitly specify update mode for models with composite primary keys

[arogachev]

For primary keys containing just one column, we can detect mode just by value - was it passed or not.

But for composite primary keys situation is more complicated because in both create and update modes we need to fully specify all primary key values so we need explicitly tell importer that we either creating or updating model.

[arogachev]

Related: https://github.com/arogachev/yii2-excel/issues/18

We can specify loaded primary keys, then the new model will be created. But sometimes we need to create new one using values replacement, so this won't work.