Add OfflineList deserialization test, fix issues

SabreTools.Models/OfflineList/CanOpen.cs

@@ -8,5 +8,17 @@ namespace SabreTools.Models.OfflineList
     {
         [XmlElement("extension")]
         public string[]? Extension { get; set; }
+
+        #region DO NOT USE IN PRODUCTION
+
+        /// <remarks>Should be empty</remarks>
+        [XmlAnyAttribute]
+        public XmlAttribute[]? ADDITIONAL_ATTRIBUTES { get; set; }
+
+        /// <remarks>Should be empty</remarks>
+        [XmlAnyElement]
+        public object[]? ADDITIONAL_ELEMENTS { get; set; }
+
+        #endregion
     }
 }