Add support for downloading files with `unknown content-length`

[celestix]

Currently the downloader doesn't support downloading files which don't specify the Content-Length HTTP Header for the body. Previously an explicit fatal error was added to the downloader for such cases to avoid unexpected circumstances with the downloader engine and it would throw this error:

The reason behind downloader engine not working with unknown content length is that, it evaluates the number of parts to download the file at starting and has no logic to deal with files with unknown length.

Counter Solution: Support for unknown content length can be added by automatically force-setting num_max_part and num_max_conn to 1 to avoid downloading it in multiple parts.