Replace spec of customer's internal logic

Specs should test public methods. Private methods should be able to
change without specs breaking.

The specs were also just mirroring the code and didn't really have any
meaning. So I wrote new ones which are hopefully clearer.

There's actually a case which I don't quite understand. Why don't we
require a customer for a new complete order? It means that we don't have
a customer creating it but then it's added when updating the order.
Seems inconsistent.

spec/models/spree/order_spec.rb

@@ -893,45 +893,27 @@ describe Spree::Order do
     end
   end
 
-  describe "#require_customer?" do
-    context "when the record is new" do
-      let(:order) { build(:order, state: state) }
-
-      context "and the state is not cart" do
-        let(:state) { "complete" }
-
-        it "returns false" do
-          expect(order.send(:require_customer?)).to eq(false)
-        end
-      end
-
-      context "and the state is cart" do
-        let(:state) { "cart" }
-
-        it "returns false" do
-          expect(order.send(:require_customer?)).to eq(false)
-        end
-      end
+  describe "#customer" do
+    it "is not required for new records" do
+      is_expected.to_not validate_presence_of(:customer)
     end
 
-    context "when the record is not new" do
-      let(:order) { create(:order, state: state) }
+    it "is not required for new complete orders" do
+      order = Spree::Order.new(state: "complete")
 
-      context "and the state is not cart" do
-        let(:state) { "complete" }
+      expect(order).to_not validate_presence_of(:customer)
+    end
 
-        it "returns true" do
-          expect(order.send(:require_customer?)).to eq(true)
-        end
-      end
+    it "is not required for existing orders in cart state" do
+      order = create(:order)
 
-      context "and the state is cart" do
-        let(:state) { "cart" }
+      expect(order).to_not validate_presence_of(:customer)
+    end
 
-        it "returns false" do
-          expect(order.send(:require_customer?)).to eq(false)
-        end
-      end
+    it "is created for existing orders in complete state" do
+      order = create(:order, state: "complete")
+
+      expect { order.valid? }.to change { order.customer }.from(nil)
     end
   end