Can't use lambda expression for MessageHandler

Neither of the following two lambda expression work, when registering a PongMessage handler.

```
session.addMessageHandler((MessageHandler.Whole) message -> {
});
```
```
session.addMessageHandler(PongMessage.class, message -> {
});
```

The reason is, that the internal method `static Class getHandlerType(MessageHandler handler)` returns `Object` instead of `PongMessage` for the type.

The workaround for this issue is to not using lambdas, but implement the interface on a class and use that:

```
class PongHandler implements MessageHandler.Whole
```