BUYLineItem didn't have a productId when initialized with BUYVariant

Update develop from master

Public/apple pay shop fix

Correct the spelling of CocoaPods in README

Also improve error description.

Adds bundle identifier to user_agent

Fix crash when presenting product view controller from tabbar

Checkout attributes

Merge branch 'runmad.checkout-attributes' of github.com:Shopify/mobile-buy-sdk-private into runmad.checkout-attributes

This adds a attributes to checkout. These objects are returned as an array of dictionaries, but expected as dictionaries when posting the checkout, so we're required to serialize the objects correctly. API now also returns the `note` on a checkout, so I have added a test for that.

Prevent race condition during Apple Pay Checkout

cleanup error handling for Apple Pay
Use dispatch_groups to ensure the shop and checkout are completed before continuing with an Apple Pay checkout

Remove shippingRateId on BUYCheckout

Add help section + section links to readme

Remove province requirement from client-side address validation

Release 1.2.4

Remove unused orderId property

Fix Apple Pay "Setup" button logic

Sample app Safari view controller checkout logic improvements

Move logic for displaying the Apple Pay setup button to the BUYViewController, and fix logic for whether to display Setup or Buy buttons.