Fix. recive 2xx while ftp.get cause timeout error

In this example:
>Client send: MDTM test.txt
>Server response: 213 20120824120909
Because FTP server do not open new channel (2XX response)
and LuaSocket try open new channel we get timeout.

```lua
local ftp   = require "socket.ftp"
local ltn12 = require "ltn12"
local url   = require("socket.url")

local URL = "ftp://USER:TEST@127.0.0.1";
local CMD = 'MDTM test.txt';

-- get timeout
ftp.get{
  url = URL;
  command = CMD;
  sink = ltn12.sink.table{};
}

-- or we can use ftp.command
ftp.command{
  url = URL;
  command = URL,
  check = function(...)
    local status, data = ...
    return true
  end;
}
```