Shield the OnDisconnect AtmosphereIntercetor from connection close

[jfarcand]

WARNING: StandardWrapperValve[AtmosphereServlet]: PWC1406: Servlet.service() for servlet AtmosphereServlet threw exception
java.lang.NullPointerException
    at org.apache.catalina.connector.Request.setAttribute(Request.java:1689)
    at org.apache.catalina.connector.RequestFacade.setAttribute(RequestFacade.java:570)
    at org.atmosphere.cpr.AtmosphereRequest.setAttribute(AtmosphereRequest.java:562)
    at org.atmosphere.interceptor.OnDisconnectInterceptor.inspect(OnDisconnectInterceptor.java:59)
    at org.atmosphere.cpr.AsynchronousProcessor.invokeInterceptors(AsynchronousProcessor.java:287)
    at org.atmosphere.cpr.AsynchronousProcessor.action(AsynchronousProcessor.java:253)
    at org.atmosphere.cpr.AsynchronousProcessor.suspended(AsynchronousProcessor.java:164)