Fix old Spree specs

Given the importance of this code, it doesn't bring me much confidence.
Apparently, this specs where using a non-existent state by mistake and
this went unnoticed because the payment creation was failing silently in
payment/processing.rb.

This unearthed the fact that our `#ensure_correct_adjustment` needs the
order to be persisted to succeed.

spec/models/spree/payment_spec.rb

@@ -2,11 +2,7 @@ require 'spec_helper'
 
 describe Spree::Payment do
   context 'original specs from Spree' do
-    let(:order) do
-      order = Spree::Order.new(:bill_address => Spree::Address.new,
-                               :ship_address => Spree::Address.new)
-    end
-
+    let(:order) { create(:order) }
     let(:gateway) do
       gateway = Spree::Gateway::Bogus.new(:environment => 'test', :active => true)
       gateway.stub :source_required => true
@@ -339,7 +335,7 @@ describe Spree::Payment do
 
       context "#credit" do
         before do
-          payment.state = 'complete'
+          payment.state = 'completed'
           payment.response_code = '123'
         end