Long-polling requests no more show XML parsing syntax error in Firefo…

…x console

Fix bug #2495

modules/cpr/src/main/java/org/atmosphere/interceptor/JavaScriptProtocol.java

@@ -178,6 +178,7 @@ public void onSuspend(AtmosphereResourceEvent event) {
                 request.setAttribute(CALLBACK_JAVASCRIPT_PROTOCOL, a);
                 r.addEventListener(a);
             } else {
+                response.setContentType("text/plain");
                 response.write(protocolMessage.get());
             }