Add simple variant proxy

spec/lib/open_food_network/variant_proxy_spec.rb

@@ -0,0 +1,27 @@
+require 'open_food_network/variant_proxy'
+
+module OpenFoodNetwork
+  describe VariantProxy do
+    let(:hub) { double(:hub) }
+    let(:v)   { double(:variant, sku: 'sku123', price: 'global price') }
+    let(:vp) { VariantProxy.new(v, hub) }
+
+    describe "delegating calls to proxied variant" do
+      it "delegates sku" do
+        vp.sku.should == 'sku123'
+      end
+    end
+
+    describe "looking up the price" do
+      it "returns the override price when there is one" do
+        VariantOverride.stub(:price_for) { 'override price' }
+        vp.price.should == 'override price'
+      end
+
+      it "returns the global price otherwise" do
+        VariantOverride.stub(:price_for) { nil }
+        vp.price.should == 'global price'
+      end
+    end
+  end
+end