Utf8Json.UniversalCodeGenerator drops the @ character prefixies

[shiena]

Step to reproduction

1. Create a class like the following

   [System.Serializable]
   public class Model
   {
       public string @event;
   }

2. Generate the source with the following command

   Utf8Json.UniversalCodeGenerator -i Model.cs -o Utf8JsonGenerated.cs

3. The @ is missing from the variable name in Utf8JsonGenerated.cs. This causes a compile error.