ftp: attempt to work-around pureftp sending spurious 150 messages

pureftpd has a bug where it sends messages like this

```
    150-Accepted data connection\r\n
        Response code: File status okay; about to open data connection (150)
        Response arg: Accepted data connection
    150 32768.0 kbytes to download\r\n
    150 0.014 seconds (measured here), 1665.27 Mbytes per second\r\n
```

The last `150` is treated as a new response - the previous `150` should have been `150-`.

This means that rclone sees the `150 0.014 seconds (measured here),
1665.27 Mbytes per second` as a reply to the next message and reports
it as an error.

This fix ignores that specific message when it is received in the
`Close` method. It dumps the FTP connection after as it is out of
sync.

See: #3984
Fixes #3445

backend/ftp/ftp.go

@@ -799,10 +799,12 @@ func (f *ftpReadCloser) Close() error {
 		f.f.putFtpConnection(&f.c, nil)
 	}
 	// mask the error if it was caused by a premature close
+	// NB StatusAboutToSend is to work around a bug in pureftpd
+	// See: https://github.com/rclone/rclone/issues/3445#issuecomment-521654257
 	switch errX := err.(type) {
 	case *textproto.Error:
 		switch errX.Code {
-		case ftp.StatusTransfertAborted, ftp.StatusFileUnavailable:
+		case ftp.StatusTransfertAborted, ftp.StatusFileUnavailable, ftp.StatusAboutToSend:
 			err = nil
 		}
 	}