Fix CustomConverter conversion lookup.

[christophstrobl]

The change (be8e702) introduced a more concrete converter lookup which causes trouble using the underlying ConversionService that might not have the converter registered for the given type but one of its parent ones. This leads to a ConverterNotFoundException during the mapping process.

interface Message<T> {
	T getValue();
}

class GenericMessage<T> implements Message<T> {
	T value;
}

class TypedMessage extends GenericMessage<String> {
}

@ReadingConverter
class MessageConverter implements Converter<Document, Message<?>> {
	@Override
	public Message<?> convert(org.bson.Document source) {
		// ...
	}
}

// ...

Document source = new Document("_class", TypedMessage.class.getName())
		.append("value", "v1");

Message<?> target = converter.read(Message.class, source);