Fix issue #7526. Reload the association target if it's stale.

* Fix issue #7526. The target's state is stale, but is not reloaded.

* This has been fixed at master via [365b8b6](https://github.com/rails/rails/commit/365b8b6db750151b786b0a7ef9e65a6824576f1b), but not at 3-2-stable branch.

* `@stale_state` should be nil when a model isn't saved. via [0f3901e](https://github.com/rails/rails/commit/0f3901e9101837eaab2daba5b01f67ed7e2c75d5).

* set `@stale_state` to nil when reset the target.