Skip methods that distributor can't use in Package

This ensures that Spree's shipping methods take into account the ones
the order's distributor uses.

This reimplements what `#available_to_order_with_distributor_check?`
used to do and it doesn't work with Spree 2.0.4.

spec/models/stock/package_spec.rb

@@ -0,0 +1,51 @@
+require 'spec_helper'
+
+module Stock
+  describe Package do
+    describe '#shipping_methods' do
+      let(:stock_location) { double(:stock_location) }
+      let(:order) { build(:order) }
+
+      subject(:package) { Package.new(stock_location, order, contents) }
+
+      describe '#shipping_methods' do
+        let(:enterprise) { build(:enterprise) }
+        let(:other_enterprise) { build(:enterprise) }
+
+        let(:order) { build(:order, distributor: enterprise) }
+
+        let(:variant1) do
+          instance_double(
+            Spree::Variant,
+            shipping_category: shipping_method1.shipping_categories.first
+          )
+        end
+        let(:variant2) do
+          instance_double(
+            Spree::Variant,
+            shipping_category: shipping_method2.shipping_categories.first
+          )
+        end
+        let(:variant3) do
+          instance_double(Spree::Variant, shipping_category: nil)
+        end
+
+        let(:contents) do
+          [
+            Package::ContentItem.new(variant1, 1),
+            Package::ContentItem.new(variant1, 1),
+            Package::ContentItem.new(variant2, 1),
+            Package::ContentItem.new(variant3, 1)
+          ]
+        end
+
+        let(:shipping_method1) { create(:shipping_method, distributors: [enterprise]) }
+        let(:shipping_method2) { create(:shipping_method, distributors: [other_enterprise]) }
+
+        it 'does not return shipping methods not used by the package\'s order distributor' do
+          expect(package.shipping_methods).to eq [shipping_method1]
+        end
+      end
+    end
+  end
+end