fix(database): use db() syntax rather than pulling dbName from client

Author: vkarpov15

lib/agenda/database.js

@@ -27,7 +27,6 @@ module.exports = function(url, collection, options, cb) {
   collection = collection || 'agendaJobs';
   options = Object.assign({autoReconnect: true, reconnectTries: Number.MAX_SAFE_INTEGER, reconnectInterval: this._processEvery}, options);
   MongoClient.connect(url, options, (error, client) => {
-    const database = client.s.options.dbName || 'agenda';
     if (error) {
       debug('error connecting to MongoDB using collection: [%s]', collection);
       if (cb) {
@@ -39,7 +38,7 @@ module.exports = function(url, collection, options, cb) {
     }
     debug('successful connection to MongoDB using collection: [%s]', collection);
     self._db = client;
-    self._mdb = client.db(database);
+    self._mdb = client.db();
     self.db_init(collection, cb);
   });
   return this;