[Bugfix BETA] App.destroy resets routes before destroying the container.

While testing it is common to create an App during setup and destroy it during
teardown.  To prevent ordering issues, routes should be destroyed before the
container is destroyed.  This behavior already exists in App.reset but should
also exist in App.destroy.
Merge pull request #4290 from jandet/willDestroy_reset_router

[Bugfix BETA] App.destroy resets routes before destroying the container.