Refactored ActiveRecord::find() so that find(null) will also return an ActiveRecord instance instead of ActiveQuery.

issue #2852

Fixes #2653: Fixed the bug that unsetting an unpopulated AR relation would trigger exception (qiangxue)

fixes #2623

allow extending only one class to add scopes, fixes #2146

TODO:

- [ ] adjust guide docs
- [ ] adjust README files of extensions
- [ ] finish work and fix test breaks

fixes #1469