Issue #7086 - Demand should not throw if the WebSocket session is closed.

Signed-off-by: Lachlan Roberts <lachlan@webtide.com>

jetty-websocket/websocket-core-common/src/main/java/org/eclipse/jetty/websocket/core/internal/WebSocketCoreSession.java

@@ -422,10 +422,6 @@ public class WebSocketCoreSession implements IncomingFrames, CoreSession, Dumpab
     {
         if (!demanding)
             throw new IllegalStateException("FrameHandler is not demanding: " + this);
-
-        if (!sessionState.isInputOpen())
-            throw new IllegalStateException("FrameHandler input not open: " + this);
-
         connection.demand(n);
     }