Workaround FTP servers reporting the wrong file size

[ismaell]

As reported by @makunterry, the SIZE command is returning an incorrect value.

Example session

$ axel ftp://ftp.info-zip.org/pub/infozip/src/unzip60.tgz
Initializing download: ftp://ftp.info-zip.org/pub/infozip/src/unzip60.tgz
File size: 1377715 bytes
Opening output file unzip60.tgz
Starting download
<...>

Related-to: https://github.com/axel-download-accelerator/axel/issues/209