abstract payment tokens
add unit tests for apple pay xml injection
add placeholder remote tests for auth.net apple pay transactions, fix whitespace
auth_net should take care of encoding apple pay token as needed
replace Authorize.Net credentials with Apple Pay-enabled ones
move apple_pay_payment_token into Auth.Net remote tests, since all data is specific to Auth.Net, simplify apple pay token tests
review items: mention freshness of test credentials, realistic instantiation of ApplePayToken objects in test, style polish
Merge pull request #1437 from Shopify/authorize_net_apple_pay

Payment Tokens, Apple Pay Payment Tokens, Authorize.Net Apple Pay support