Fix conn leak in Quit(). Closes #13.

Co-authored-by: TestMsr <cs3120@vip.qq.com>

pop3.go

@@ -419,10 +419,13 @@ func (c *Conn) Noop() error {
 // Message deletions (DELE command) are only excuted by the server on a graceful
 // quit and close.
 func (c *Conn) Quit() error {
+	defer c.conn.Close()
+
 	if _, err := c.Cmd("QUIT", false); err != nil {
 		return err
 	}
-	return c.conn.Close()
+
+	return nil
 }
 
 // parseResp checks if the response is an error that starts with `-ERR`