don't overwrite RNG

[ExpandingMan]

Checklist

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

Additional context

This is a suggested fix for #249. I'm getting test failures that seem unrelated. Most (all?) existing tests use Random.seed! so this shouldn't affect those one way or another. We can either add tests here or change some of the Random.seed!.

Technically I suppose this is slightly breaking.

[ChrisRackauckas]

I think this would be highly breaking. All MTK SDEs for example would no longer be random.