Merge quantity of units when serializing shipment.items

Currently when receiving that shipment back via a webhook from wombat
the extention would complain about a diff in the items.

e.g. The received shipment items do not match with the shipment, diff: [{:sku=>"123", :quantity=>1}, {:sku=>"123", :quantity=>1}]

Before:

        items: [{ product_id: 1, quantity: 1 }, { product_id: 1, quantity: 1 }]

Now:

        items: [{ product_id: 1, quantity: 2 }]

app/serializers/spree/wombat/inventory_unit_serializer.rb

@@ -10,7 +10,8 @@ def quantity
       end
 
       def price
-        object.line_item.price.round(2).to_f
+        price = object.respond_to?(:price) ? object.price : object.line_item.price
+        price.round(2).to_f
       end
 
       def product_id

app/serializers/spree/wombat/shipment_serializer.rb

@@ -71,8 +71,14 @@ def updated_at
 
       def items
         i = []
-        object.inventory_units.each do |inventory_unit|
-          i << InventoryUnitSerializer.new(inventory_unit, root: false)
+        object.inventory_units.group_by(&:line_item).each do |line_item, units|
+          line = LineItem.new do |line|
+            line.quantity = units.count
+            line.variant = line_item.variant
+            line.price = line_item.price
+          end
+
+          i << InventoryUnitSerializer.new(line, root: false)
         end
         i
       end

spec/serializers/spree/wombat/shipment_serializer_spec.rb

@@ -7,6 +7,30 @@ module Wombat
       let(:shipment) { create(:shipment, address: create(:address), order: create(:order_with_line_items)) }
       let(:serialized_shipment) { JSON.parse (ShipmentSerializer.new(shipment, root: false).to_json) }
 
+      it "merge inventory into a single shipment item when needed" do
+        variant = Variant.new sku: "wom"
+        line = LineItem.new variant: variant
+        inventories = 2.times.map {
+          InventoryUnit.new line_item: line, variant: variant
+        }
+
+        shipment = Shipment.new
+        shipment.inventory_units = inventories
+
+        serialized = ShipmentSerializer.new(shipment, root: false)
+        expect(serialized.items.first.quantity).to eq 2
+
+        other_variant = Variant.new sku: "other wom"
+        other_line = LineItem.new variant: other_variant
+        inventories.push InventoryUnit.new(line_item: other_line, variant: other_variant)
+
+        shipment = Shipment.new
+        shipment.inventory_units = inventories
+
+        serialized = ShipmentSerializer.new(shipment, root: false)
+        expect(serialized.items.last.quantity).to eq 1
+      end
+
       it "serializes the number as id" do
         expect(serialized_shipment["id"]).to eql shipment.number
       end