Handle return code 125 for RETR command

While working with a particular FTP server, the server returned 125 instead of 150 while getting a file in response to the RETR command. Turns out that's a valid response code as well. Allowing the response processing code for RETR to handle 125 as well moves things along nicely - otherwise the code just waits indefinitely for a 150 to show up which never comes.