Add Hashfile deserialization test, fix issues

SabreTools.Models/Hashfile/Hashfile.cs

@@ -16,5 +16,12 @@ namespace SabreTools.Models.Hashfile
         public SHA512[]? SHA512 { get; set; }
 
         public SpamSum[]? SpamSum { get; set; }
+
+        #region DO NOT USE IN PRODUCTION
+
+        /// <remarks>Should be empty</remarks>
+        public string[]? ADDITIONAL_ELEMENTS { get; set; }
+
+        #endregion
     }
 }