The patch looks good to me.
Only two little remarks:

1: I'd create two new separate tests rather than appending them to
existent ones.

2: > self.client.retrbinary('retr', received.append, rest=str(rest))

str() should be useless here.

> According to the RFC, the argument to REST can be any string of
> printable characters. However, does it happen for clients to put
> non-digits in there?

It shouldn't happen but in any case I woulnd't want ftplib to check for
such a kind of thing.
Deciding whether the REST argument is invalid is up to the server, in
which case it will send a 4xx/5xx error response.

> But I've tested some ftp servers and they don't return an
> error code, they just set REST to 0 and return some OK code.

IMHO a bad design choice.