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

* This has been fixed at master via `365b8b6`, but not at 3-2-stable branch.

* @stale_state should be nil when a model isn't saved. via `0f3901e`.

* set @stale_state to nil when reset the target.
Merge pull request #9549 from larrylv/reload-stable-target-before-saving

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