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

test/gelfAppender-test.js

@@ -244,6 +244,7 @@ vows.describe('log4js gelfAppender').addBatch({
       },
       'should pick up the options': function(message) {
         assert.equal(message.host, 'cheese');
+        assert.equal(message.GELF, void(0)); // make sure flag was removed
         assert.equal(message._facility, 'nonsense');
         assert.equal(message._every1, 'Hello every one'); // the default value
         assert.equal(message._every2, 'Overwritten!'); // the overwritten value