Use default AR _update_record unless the object uses a composite key

This came up in reference to https://github.com/rails/rails/pull/14971

When changes are made to Active Record's version of _update_record, they are not available for those using this gem. If there is a better way to fix this, please let me know and I'll see if I can work it up.

I don't think any new tests are necessary, but if you think there should be some I'll write those up as well.

Thanks for considering the change.