Perform mail delivery in spec to make it pass

We need to purposefully enable deliveries for the deliveries array to
list this email. Otherwise the delivery logic is not executed and thus
the deliveries array never filled.

spec/controllers/spree/admin/orders_controller_spec.rb

@@ -182,7 +182,9 @@ describe Spree::Admin::OrdersController, type: :controller do
           before { distributor.update_attribute(:abn, "123") }
           before do
             Spree::Config[:mails_from] = "spree@example.com"
+            ActionMailer::Base.perform_deliveries = true
           end
+
           it "should allow me to send order invoices" do
             expect do
               spree_get :invoice, params