Always recreate loadingPromise when calling retryLoading

[bg-stripe]

r? @danj-stripe

Calling retryLoading currently does nothing if the promise has been completed with a value. The method was initially intended for use when PaymentContext's initial load fails, but you would also expect it to be useful for triggering a refresh of the stored customer object:

customerContext.clearCachedCustomer()
paymentContext.retryLoading()
paymentContext.presentPaymentMethodsViewController()

I can't think of any unintended side effects from this change. The method is only called once internally, in the STPPaymentContext initialization path. Apps that use this method to recover from initial loading errors won't be affected by the change.

[bg-stripe]

note from IRL discussion: I'll update this so that retryLoading also clears the cached customer on CustomerContext

[danj-stripe]

Looks great!