Window close events randomly failing to be processed due to dead AtmosphereResource

[markathomas]

Window close events are randomly not being processed if AtmosphereResource was dead when executing the the line

String mode = resource.getRequest().getParameter(Constants.CLIENT_SERIALZE_MODE_PARAMETER);

in AtmosphereGwtHandler. In this scenario an exception is thrown and the data is ignored. Even if a resource is dead we still need to allow the data to be processed; otherwise, we may ignore important events (e.g. window close)

Pull request: #618