C# does not generate correct code for constant presence enums

[mjpt777]

I noticed the C# generator does not handle constant presence enums. This was discovered to when trying to evaluate the impact of #560. @JPWatson, @ZachBray, @billsegall may be worth taking a look.

See the generated code for the discountedModel in the car example.

        public const int DiscountedModelId = 8;
        public const int DiscountedModelSinceVersion = 0;
        public const int DiscountedModelDeprecated = 0;
        public bool DiscountedModelInActingVersion()
        {
            return _actingVersion >= DiscountedModelSinceVersion;
        }

        public static string DiscountedModelMetaAttribute(MetaAttribute metaAttribute)
        {
            switch (metaAttribute)
            {
                case MetaAttribute.Epoch: return "";
                case MetaAttribute.TimeUnit: return "";
                case MetaAttribute.SemanticType: return "";
                case MetaAttribute.Presence: return "constant";
            }

            return "";
        }

        public Model DiscountedModel
        {
            get
            {
                return (Model)_buffer.CharGet(_offset + 39);
            }
            set
            {
                _buffer.CharPut(_offset + 39, (byte)value);
            }
        }