Do not handle network error in SetCloseHandler()

The 666c197 added an error handling in `SetCloseHandler()` and peer
stops getting `CloseError` when network issue like `write: broken
pipe` happens because the close handle returns the error.

Hence this patch changes to skip network error handling.

conn.go

@@ -1164,6 +1164,9 @@ func (c *Conn) SetCloseHandler(h func(code int, text string) error) {
 		h = func(code int, text string) error {
 			message := FormatCloseMessage(code, "")
 			if err := c.WriteControl(CloseMessage, message, time.Now().Add(writeWait)); err != nil {
+				if _, ok := err.(net.Error); ok {
+					return nil
+				}
 				return err
 			}
 			return nil