save du and k for DAEs when dense==true

[oscardssmith]

Checklist

• [ ] Appropriate tests were added
• [x] Any code changes were done in a way that does not break public API
• [x] All documentation related to code changes were updated
• [x] The new code follows the contributor guidelines, in particular the SciML Style Guide and COLPRAC.
• [x] Any new documentation only uses public API

Additional context

Add any other context about the problem here. depends on https://github.com/SciML/SciMLBase.jl/pull/622 to add the k field to DAESolution.

[ChrisRackauckas]

Missing tests

[ChrisRackauckas]

Bump for tests