Xstream field with converter is not an attribute even with @XStreamAsAttribute

I try to marshal an object and I want all the fields to be attributes. The normal fields are OK with the @XStreamAsAttribute annotation but I have two of them with a converter. For them when I marshal they are converted as field...

@XStreamAlias(value="sinistre")
public class ObjetMetierSinistreDto {

    @XStreamAlias(value="S_sinistreEtat")
    @XStreamAsAttribute
    private String etat;

    @XStreamAsAttribute
    @XStreamAlias(value="S_sinistreDateSurv")
    @XStreamConverter(value=JodaDateConverter.class)
    private LocalDate dateSurvenanceDossier;
...

The converter:

public class JodaDateConverter implements Converter {

@Override
@SuppressWarnings("unchecked")
public boolean canConvert(final Class type) {
      return (type != null) && LocalDate.class.getPackage().equals(type.getPackage());
}

@Override
public void marshal(final Object source, final HierarchicalStreamWriter writer,
      final MarshallingContext context) {
      writer.setValue(source.toString().replace("-", "/"));
}

@Override
@SuppressWarnings("unchecked")
public Object unmarshal(final HierarchicalStreamReader reader,
      final UnmarshallingContext context) {
      try {
              final Class requiredType = context.getRequiredType();
              final Constructor constructor = requiredType.getConstructor(Object.class);
              return constructor.newInstance(reader.getValue());
      } catch (final Exception e) {
          throw new RuntimeException(String.format(
           "Exception while deserializing a Joda Time object: %s", context.getRequiredType().getSimpleName()), e);
      }
}

}

and the result:

<sinistre S_sinistreEtat="S">        
  <S_sinistreDateSurv>2015/02/01</S_sinistreDateSurv>
</sinistre>

and what I like:

<sinistre S_sinistreEtat="S"
          S_sinistreDateSurv="2015/02/01"/>