fix default Exec method

db/db.go

@@ -397,6 +397,6 @@ func Exec(query string, args ...interface{}) (LastInsertId, RowsAffected int64,
 		return
 	}
 
-	LastInsertId, RowsAffected, err = defaultDB.conn.Exec(query, args...)
+	LastInsertId, RowsAffected, err = defaultDB.Exec(query, args...)
 	return
 }