WebSocketResponse#sendError throws UnsupportedOperationException

Hi,

Using 7.0.0-SNAPSHOT, I've got a UnsupportedOperationException (see below)
whereas I do not have it on 7.0.0-M3. I don't really know how to
investigate as an UnsupportedOperationException seems to be the expected
result [1], or maybe #sendError is not supposed to be called anyway, or it
is a regression from -M3...

I'm using wicket-native-websocket-core wicket-native-websocket-javax on
WildFly8

Thanks in advance for your advise,
Sebastien.

[1]
https://github.com/apache/wicket/blob/master/wicket-native-websocket/wicket-native-websocket-core/src/main/java/org/apache/wicket/protocol/ws/api/WebSocketResponse.java#L193