Allow editing of invalid quantities

If the user entered an invalid quantity, Angular set the model to
undefined and we removed the input field to show the add button. That
makes it impossible for a user to see what the maximum quantity to enter
would be. For example:

- The variant has a stock level of 5.
- The user enters 7.
- Angular sets it to undefined.
- The input field disappears.
- The user is startled and doesn't know how to proceed.

But now we hide the input only if it's deliberately set to zero.

app/assets/javascripts/darkswarm/controllers/shop_variant_controller.js.coffee

@@ -6,6 +6,8 @@ Darkswarm.controller "ShopVariantCtrl", ($scope, $modal, Cart) ->
     return if old_value[0] == null && new_value[0] == null
     Cart.adjust($scope.variant.line_item)
 
+  $scope.variant.line_item.quantity ||= 0
+
   if $scope.variant.product.group_buy
     $scope.$watch "variant.line_item.quantity", ->
       item = $scope.variant.line_item

app/assets/javascripts/templates/partials/shop_variant_no_group_buy.html.haml

@@ -1,11 +1,11 @@
 .small-5.medium-3.large-3.columns.variant-quantity-column.text-right{"ng-if" => "::!variant.product.group_buy"}
 
-  .variant-quantity-inputs{ng: {if: "!variant.line_item.quantity"}}
+  .variant-quantity-inputs{ng: {if: "variant.line_item.quantity == 0"}}
     %button.add-variant{type: "button", ng: {click: "add(1)", disabled: "!canAdd(1)"}}
       {{ "js.shopfront.variant.add_to_cart" | t }}
 
-  .variant-quantity-inputs{ng: {if: "variant.line_item.quantity"}}
-    %button.variant-quantity{type: "button", ng: {click: "add(-1)"}}>
+  .variant-quantity-inputs{ng: {if: "variant.line_item.quantity != 0"}}
+    %button.variant-quantity{type: "button", ng: {click: "add(-1)", disabled: "!canAdd(-1)"}}>
       -# U+FF0D Fullwidth Hyphen-Minus
       －
     %input.variant-quantity{type: "number", min: "0", max: "{{ available() }}", ng: {model: "variant.line_item.quantity"}}>

spec/javascripts/unit/darkswarm/controllers/checkout/shop_variant_controller_spec.js.coffee

@@ -12,12 +12,15 @@ describe "ShopVariantCtrl", ->
         on_demand: true
         product: {group_buy: true}
         line_item: {
-          quantity: 0
-          max_quantity: 0
+          quantity: undefined
+          max_quantity: undefined
         }
       }
       ctrl = $controller 'ShopVariantCtrl', {$scope: scope, $modal: $modal, Cart: null}
 
+  it "initializes the quantity for shop display", ->
+    expect(scope.variant.line_item.quantity).toEqual 0
+
   it "adds an item to the cart", ->
     scope.add 1
     expect(scope.variant.line_item.quantity).toEqual 1