Do not initialize Atmosphere for websocket servlet in Websphere (#16354)

Change-Id: I6f32668d357c2f2af75a2d53de37708f6c449b44

server/src/com/vaadin/server/communication/JSR356WebsocketInitializer.java

      */
     protected boolean isVaadinServlet(ServletRegistration servletRegistration) {
         try {
-            Class<?> servletClass = Class.forName(servletRegistration
-                    .getClassName());
+            String servletClassName = servletRegistration.getClassName();
+            if (servletClassName.equals("com.ibm.ws.wsoc.WsocServlet")) {
+                // Websphere servlet which implements websocket endpoints,
+                // dynamically added
+                return false;
+            }
+            Class<?> servletClass = Class.forName(servletClassName);
             return VaadinServlet.class.isAssignableFrom(servletClass);
         } catch (Exception e) {
             // This will fail in OSGi environments, assume everything is a