hat-trick caches old/stale model variables

[paulschreiber]

Sometimes hat-trick caches old/stale model variables.

To reproduce:

1. In register_eday_controller, change redirect_to_external_url eday_wizard_register_index_url(step: :address) to next_step :address.
2. Enter a name and email
3. Enter an address in NY or NJ
4. Choose Write & publish rdoc #2 (I want to vote near my current location)
5. Enter another address
6. Press continue
7. Map shows old address