Remove GELF flag when capturing custom fields

* Logstash (which supports GELF) drops messages with this key
* There's no particular need to keep this key

lib/appenders/gelf.js

@@ -85,6 +85,8 @@ function gelfAppender (layout, host, port, hostname, facility) {
     var firstData = data[0];
     
     if (!firstData.GELF) return; // identify with GELF field defined
+    // Remove the GELF key, some gelf supported logging systems drop the message with it
+    delete firstData.GELF;
     Object.keys(firstData).forEach(function(key) {
       // skip _id field for graylog2, skip keys not starts with UNDERSCORE
       if (key.match(/^_/) || key !== "_id") {