Expose the numeric code of the Response.StatusCode

[Andremoniy]

I propose a small improvement in order to improve debugging of the errors happen during the connection.
Currently the Response.StatusCode doesn't expose its internal numeric code, which is essentially an error code returned by an SFTP server according to the specification. So it would be nice to be able to read this code directly from the StatusCode:

   public enum StatusCode {
       ...
       public int getCode() { 
            return code;
       }
    }

Without doubts one can use an enum constant name, however it doesn't directly reflect the original error code received from the SFTP server according to the SFTP specification.

[hierynomus]

Thanks for the PR, merged!