Address.state_id not cleared when changed to a country without states

[mleglise]

To reproduce:

• /checkout/address: enter United States, select a state, and fill out address, click "Save and Continue"
• from /checkout/delivery, click link to go back to /checkout/address
• select new country (ex: Chile), which hides the state select box.
• "Save and Continue" does not submit the state_id, but the value is preserved in the spree_address model instance.

This can cause problems with making certain shipping methods available (if they are available based on State-based Zones) when they are not meant for that country. Example: Given Free US Shipping for some US states, now customer can get free shipping to Chile.