[api] Expose `timestamp` property

lib/winston-mongodb.js

@@ -76,7 +76,8 @@ MongoDB.prototype.log = function (level, msg, meta, callback) {
         self.emit('error', err);
       }
 
-      var entry = { 
+      var entry = {
+        timestamp: new Date(), // RFC3339/ISO8601 format instead of common.timestamp()
         level: level, 
         message: msg, 
         meta: meta