smtp: ignore HELO error in QUIT

Merge from net/smtp/smtp.go

Obtained from:	2ef8e41f9543478a51a0147a735e4415737de09f

smtp.go

@@ -445,9 +445,7 @@ func (c *Client) Noop() error {
 
 // Quit sends the QUIT command and closes the connection to the server.
 func (c *Client) Quit() error {
-	if err := c.hello(); err != nil {
-		return err
-	}
+	c.hello() // ignore error; we're quitting anyhow
 	_, _, err := c.cmd(221, "QUIT")
 	if err != nil {
 		return err