修正mongo模型不能连接及报setField方法不存在的问题

library/think/db/driver/mongo.php

@@ -65,7 +65,7 @@ class Mongo extends Driver
     {
         if (!isset($this->linkID[$linkNum])) {
             if (empty($config)) {
-                $config = $this->config['connection'];
+                $config = $this->config;
             }
 
             $host = 'mongodb://' . ($config['username'] ? "{$config['username']}" : '') . ($config['password'] ? ":{$config['password']}@" : '') . $config['hostname'] . ($config['hostport'] ? ":{$config['hostport']}" : '') . '/' . ($config['database'] ? "{$config['database']}" : '');
@@ -95,6 +95,8 @@ class Mongo extends Driver
             $this->initConnect($master);
         }
 
+        $db = $db?$db:$this->config['database'];
+
         try {
             if (!empty($db)) {
                 // 传人Db则切换数据库