Correct issue with per item calculator when used with shipping method

Relates to #1596

Merges #1601

core/app/models/spree/calculator/per_item.rb

@@ -11,7 +11,7 @@ def self.description
     def compute(object=nil)
       return 0 if object.nil?
       self.preferred_amount * object.line_items.reduce(0) do |sum, value|
-        if matching_products && matching_products.include?(value.product)
+        if !matching_products || matching_products.include?(value.product)
           value_to_add = value.quantity
         else
           value_to_add = 0

core/spec/models/calculator/per_item_spec.rb

@@ -2,24 +2,36 @@
 
 describe Spree::Calculator::PerItem do
   # Like an order object, but not quite...
-  let!(:product) { double("Product") }
-  let!(:line_items) { [double("LineItem", :quantity => 5, :product => product)] * 3 }
+  let!(:product1) { double("Product") }
+  let!(:product2) { double("Product") }
+  let!(:line_items) { [double("LineItem", :quantity => 5, :product => product1), double("LineItem", :quantity => 3, :product => product2)] }
   let!(:object) { double("Order", :line_items => line_items) }
-  let!(:calculable) { double("Calculable", :promotion => promotion) }
-  let!(:promotion) { double("Promotion", :rules => [double("Rule", :products => [product])]) }
+
+  let!(:shipping_calculable) { double("Calculable") }
+  let!(:promotion_calculable) { double("Calculable", :promotion => promotion) }
+
+  let!(:promotion) { double("Promotion", :rules => [double("Rule", :products => [product1])]) }
 
-  let!(:calculator) do
-    calculator = Spree::Calculator::PerItem.new(:preferred_amount => 10)
-    calculator.stub(:calculable => calculable)
-    calculator
-  end
+  let!(:calculator) { Spree::Calculator::PerItem.new(:preferred_amount => 10) }
 
   # regression test for #1414
   it "correctly calculates per item shipping" do
-    calculator.compute(object).to_f.should == 150 # 5 x 3 x 10
+    calculator.stub(:calculable => shipping_calculable)
+    calculator.compute(object).to_f.should == 80 # 5 x 10 + 3 x 10
+  end
+
+  it "correctly calculates per item promotion" do
+    calculator.stub(:calculable => promotion_calculable)
+    calculator.compute(object).to_f.should == 50 # 5 x 10
+  end
+
+  it "returns 0 when no object passed" do
+    calculator.stub(:calculable => shipping_calculable)
+    calculator.compute.should == 0
   end
 
   it "returns 0 when no object passed" do
+    calculator.stub(:calculable => promotion_calculable)
     calculator.compute.should == 0
   end