GELF appender: move facility key into a custom field if present

At the same time, don't assign a default value as the field is optional
according to the GELF spec v1.1.

lib/appenders/gelf.js

@@ -48,7 +48,9 @@ function gelfAppender (layout, host, port, hostname, facility) {
   host = host || 'localhost';
   port = port || 12201;
   hostname = hostname || require('os').hostname();
-  facility = facility || 'nodejs-server';
+  if(facility) {
+    customFields['_facility'] = facility;
+  }
   layout = layout || layouts.messagePassThroughLayout;
 
   var defaultCustomFields = customFields || {};
@@ -103,7 +105,6 @@ function gelfAppender (layout, host, port, hostname, facility) {
     msg.timestamp = msg.timestamp || new Date().getTime() / 1000; // log should use millisecond 
     msg.host = hostname;
     msg.level = levelMapping[loggingEvent.level || levels.DEBUG];
-    msg.facility = facility;
     return msg;
   }