Fix custom marshaler for enum types

When MarshalJSON was defined on a pointer receiver custom enum type
marshaling/unmarshaling was panicing since the underlying primitive type
was treated as a pointer.

Since method set for pointer receivers includes value receiver methods
we don't really need optionalEncoder and can just use marshalEncoder
directly.

feature_reflect_native.go

@@ -661,6 +661,7 @@ func (encoder *marshalerEncoder) Encode(ptr unsafe.Pointer, stream *Stream) {
 	templateInterface.word = ptr
 	realInterface := (*interface{})(unsafe.Pointer(&templateInterface))
 	marshaler := (*realInterface).(json.Marshaler)
+
 	bytes, err := marshaler.MarshalJSON()
 	if err != nil {
 		stream.Error = err