Allow orders to be paid before checkout

Taler puts the payment completion into the hands of the user. So we
can't strictly finalise the payment and order together.

And in the bigger picture, it should be okay if a payment goes through
but we have to abort checkout due to stock issues. Then we want to be
able to check out again, using the existing complete payment. Any
refunds can be handled later by the shop owner.

app/models/spree/order.rb

@@ -674,8 +674,6 @@ module Spree
     end
 
     def process_each_payment
-      raise Core::GatewayError, Spree.t(:no_pending_payments) if pending_payments.empty?
-
       pending_payments.each do |payment|
         if payment.amount.zero? && zero_priced_order?
           payment.update_columns(state: "completed", captured_at: Time.zone.now)

spec/models/spree/order_spec.rb

@@ -282,11 +282,6 @@ RSpec.describe Spree::Order do
     let(:payment) { build(:payment) }
     before { allow(order).to receive_messages pending_payments: [payment], total: 10 }
 
-    it "returns false if no pending_payments available" do
-      allow(order).to receive_messages pending_payments: []
-      expect(order.process_payments!).to be_falsy
-    end
-
     context "when the processing is sucessful" do
       it "processes the payments" do
         expect(payment).to receive(:process!)