fix get connection cursor

Author: snower

tormysql/client.py

@@ -72,12 +72,12 @@ def select_db(self, db):
     def cursor(self, cursor_cls=None):
         if cursor_cls is None:
             cursor_cls = self._connection.cursorclass
-            
+
         cursor = self._connection.cursor(
             cursor_cls.__delegate_class__ if cursor_cls and issubclass(cursor_cls, Cursor) else cursor_cls
         )
 
-        if cursor_cls:
+        if issubclass(cursor_cls, Cursor):
             return cursor_cls(cursor)
         else:
             return cursor.__tormysql_class__(cursor)