Add missing category field

Current logstashUDP appender doesn't send category of the loggingEvent which is very useful when filtering for logs.

lib/appenders/logstashUDP.js

@@ -21,7 +21,8 @@ function logstashUDP (config, layout) {
       '@timestamp': (new Date(loggingEvent.startTime)).toISOString(),
       type: type,
       message: logMessage,
-      fields: fields
+      fields: fields,
+      category: loggingEvent.logger.category
     };
     sendLog(udp, config.host, config.port, logObject);
   };