fails:Net::FTP#mkdir sends the MKD command with the passed pathname to the server
fails:Net::FTP#mkdir returns the path to the newly created directory
fails:Net::FTP#mkdir raises a Net::FTPPermError when the response code is 500
fails:Net::FTP#mkdir raises a Net::FTPPermError when the response code is 501
fails:Net::FTP#mkdir raises a Net::FTPPermError when the response code is 502
fails:Net::FTP#mkdir raises a Net::FTPTempError when the response code is 421
fails:Net::FTP#mkdir raises a Net::FTPPermError when the response code is 530
fails:Net::FTP#mkdir raises a Net::FTPPermError when the response code is 550
