Fix billing address

[maxhungry]

Proposed changes

Currently, when create payment method, the address under billing_details is not posted in iOS and Android.

iOS:

• Fix address to use STPPaymentMethodAddress.

Android:

• Fix addressParams lookup at wrong map
• Change address builder's setCountry to accepts nullable

Types of changes

What types of changes does your code introduce to tipsi-stripe?
Put an x in the boxes that apply

• Bugfix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality not to work as expected)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! Next steps are a reminder of what we are going to look at before merging your code.

• I have added tests that prove my fix is useful or that my feature works
• I have added necessary documentation (if appropriate)
• I know that my PR will be merged only if it has tests and they pass

[DaJaffaMan]

yeah we just spotted this one today as our customer billing details weren't being stored when saving their card payments, can this get merged any time soon?

[fbartho]

This code looks great! -- Pretty straightforward bug in copy-paste, and since not every customer uses Addresses, I guess we missed it until now.

Since my product doesn't use it -- who has tested this code in their app?

I'm comfortable merging this because the code looks like clear copy-paste-type-mistakes, and the code uses appropriate helpers to handle nil/parsing.

If somebody has tested this, we can merge and release a new version!

[cybergrind]

@fbartho in that case we're good to merge it