Convert jQuery data attribute to number

JQuery numeric string data attributes are not casted to numbers
consistently. There are instances when they are kept as string,
this behavior seems to be related to the presence of insignificant
zeros in the original string:

  $('<div data-price="12.00"></div>').data('price')
  > "12.00"

  $('<div data-price="12.10"></div>').data('price')
  > "12.10"

but:

  $('<div data-price="12.1"></div>').data('price')
  > 12.1

So, we need to enforce the typecast to number when summing those
attributes in the function `totalSelectedReturnItemAmount`.

After the sum, the number needs to be converted to a properly
localized amount. For this we leverage the existing function
`formatNumber` from the library `accounting.js`, see

https://github.com/solidusio/solidus/blob/master/backend/vendor/assets/javascripts/solidus_admin/accounting.js

backend/app/assets/javascripts/spree/backend/views/tables/selectable_table/sum_return_item_amount.js

@@ -15,18 +15,25 @@ Spree.Views.Tables.SelectableTable.SumReturnItemAmount = Backbone.View.extend({
   },
 
   totalSelectedReturnItemAmount: function () {
-    var totalAmount = 0.00;
+    var totalAmount = 0;
     var selectedItems = [];
+    var decimals = 0;
+    var separator = Spree.t('currency_separator');
+    var amount, decimalAmount;
 
     if(this.model.get('allSelected')) {
       selectedItems = $('.selectable');
     } else {
       selectedItems = $(this.model.attributes.selectedItems);
     }
     selectedItems.each(function(_, selectedItem){
-      totalAmount += $(selectedItem).data('price');
+      amount = $(selectedItem).data('price');
+      decimalAmount = amount.toString().split(separator)[1] || '';
+      decimals = Math.max(decimals, decimalAmount.length);
+
+      totalAmount += parseFloat(amount);
     })
 
-    return totalAmount;
+    return accounting.formatNumber(totalAmount, decimals, Spree.t('currency_delimiter'), separator);
   },
 });