修复ResponseMessage.onlyData 为string的时候，被转为json的问题

hsweb-web-core/src/main/java/org/hsweb/web/core/message/FastJsonHttpMessageConverter.java

@@ -87,7 +87,10 @@ public class FastJsonHttpMessageConverter extends AbstractHttpMessageConverter<O
             ResponseMessage message = (ResponseMessage) obj;
             if (message.isSuccess() && message.isOnlyData())
                 obj = message.getData();
-            text = JSON.toJSONString(obj, parseFilter(message), features);
+            if (obj instanceof String)
+                text = ((String) obj);
+            else
+                text = JSON.toJSONString(obj, parseFilter(message), features);
             if (callback == null) callback = message.getCallback();
         } else {
             text = JSON.toJSONString(obj, features);