More checks around zip code
More checks around zip code

Do some more guarding around geocoding to make sure:
1. we aren't firing off more than one simulataneously (which the docs warn against)
2. zip code hasnt changed while geocode is in flight
3. We aren't asking for nil values

stripe/stripe-ios,stripe/stripe-ios,stripe/stripe-ios,stripe/stripe-ios,stripe/stripe-ios,stripe/stripe-ios