Restart the transfer when content-lenght is 0, fixes #275

Instead of adding to the same file between each stop/resume, the transfer is restarded when the server do not provide a content-length.

(please be careful when integrating this fix, I'm really rusty at coding)
Merge pull request #277 from Cqoicebordel/patch-2

Restart the transfer when content-lenght is 0, fixes #275